I have a built in mic and the computer says that it can't not find it. I have also checked to make sure that my computer is using the default setting are there any other ideas?

You do not say what sort of mic you have - if its a built in laptop mic, a standalone mic or one built into the webcam.
It would also be useful to know what your OS is and what chat program you want to use the mic in
However we can generalise and hopefully put you on the right track

Have you set the mic you wish to use with your webcam as Default ?

XP
Plug in your webcam or headset or mic you wish to use with your webcam
Control Panel (Classic View)
Open 'Sounds and Audio Devices' folder and click on the 'Volume' tab - make sure the speakers are not muted, open 'Speaker volume' and set the sliders toward the upper end.
Open 'Audio' tab - make your preferred mic the default device in the 'Sound recording window'
Open 'Volume' below the window to ensure the mic is not muted
Click 'Apply' and 'OK'
Restart your computer
Configure the webcam / mic to your chat program

VISTA
Plug in your webcam or headset or mic you wish to use with your webcam
TEST SPEAKERS
Right click on speaker icon in System Tray or from Control Panel
Select "Playback Devices"
The speaker icon ( checked as working) and volume level graph should be visible
Right click on the volume graph and click "Test"
Left hand speakers followed by right hand should be heard

SET DEFAULT & TEST MIC
Click "Recording" tab.
Right click on the volume graph and select 'Show disabled devices' and 'Show disconnected devices'
To set your preferred mic as default - click the mic font to highlight and click "Properties"
In lower dropbox display "Use this device enable" click 'OK'
Click "Levels" tab and ramp up sliders ( adjust to suit later)
Return to "Recording". Test mic by speaking and make sure volume graph displays

Restart computer

Configure your webcam and mic to the Chat program

If you have a mic built in to your webcam you have to make it your default mic. Especially if you also have a laptop with a built in mic.
If your OS is pre-Vista you will have to set your webcam mic as default from the Control Panel - make sure it's plugged in

Vista
Plug in the webcam
TEST SPEAKERS
Right click on speaker icon in System Tray or from Control Panel
Select "Playback Devices"
The speaker icon ( checked as working) and volume level graph should be visible
Right click on the volume graph and click "Test"
Left hand speakers followed by right hand should be heard

SET DEFAULT & TEST MIC
Click "Recording" tab.
Right click on the volume graph and select 'Show disabled devices' and 'Show disconnected devices'
To set required (webcam) mic as default - click on mic font to highlight and click "Properties"
In lower dropbox display "Use this device enable" click 'OK'
Click "Levels" tab and ramp up sliders ( adjust to suit later)
Return to "Recording". Test mic by speaking and make sure volume graph displays

Restart computer

Not all webcams have a built in mic, but if yours has one set it as default
TEST SPEAKERS
Right click on speaker icon in System Tray or from Control Panel
Select "Playback Devices"
The speaker icon ( checked as working) and volume level graph should be visible
Right click on the volume graph and click "Test"
Left hand speakers followed by right hand should be heard

SET DEFAULT & TEST MIC
Click "Recording" tab.
To set required (webcam) mic as default - click on mic font to highlight and click "Properties"
In lower dropbox display "Use this device enable" click 'OK'
Click "Levels" tab and ramp up sliders ( adjust to suit later)
Return to "Recording". Test mic by speaking and make sure volume graph displays

Restart computer

You may have a laptop with a built in mic or headphones with built in mic and webcam with built in mic
You need to set your webcam mic as default
Vista
TEST SPEAKERS
Right click on speaker icon in System Tray or from Control Panel
Select "Playback Devices"
The speaker icon ( checked as working) and volume level graph should be visible
Right click on the volume graph and click "Test"
Left hand speakers followed by right hand should be heard

SET DEFAULT & TEST MIC
Click "Recording" tab.
To set required (webcam) mic as default - click on mic font to highlight and click "Properties"
In lower dropbox display "Use this device enable" click 'OK'
Click "Levels" tab and ramp up sliders ( adjust to suit later)
Return to "Recording". Test mic by speaking and make sure volume graph displays

Restart computer

Configure to your chat program

make sure the webcam mic is set as default
TEST SPEAKERS
Right click on speaker icon in System Tray or from Control Panel
Select "Playback Devices"
The speaker icon ( checked as working) and volume level graph should be visible
Right click on the volume graph and click "Test"
Left hand speakers followed by right hand should be heard

SET DEFAULT & TEST MIC
Click "Recording" tab.
To set required (webcam) mic as default - click on mic font to highlight and click "Properties"
In lower dropbox display "Use this device enable" click 'OK'
Click "Levels" tab and ramp up sliders ( adjust to suit later)
Return to "Recording". Test mic by speaking and make sure volume graph displays

Restart computer

Make sure you have your webcam mic set as default - especially if you have a laptop with a built in mic
TEST SPEAKERS
Right click on speaker icon in System Tray
Select "Playback Devices"
The speaker icon ( checked as working) and volume level graph should be visible
Right click on the volume graph and click "Test"
Left hand speakers followed by right hand should be heard

SET DEFAULT & TEST MIC
Click "Recording" tab.
To set required (webcam) mic as default - click on mic font to highlight and click "Properties"
In lower dropbox display "Use this device enable" click 'OK'
Click "Levels" tab and ramp up sliders ( adjust to suit later)
Return to "Recording". Test mic by speaking and make sure volume graph displays

Restart computer

If your webcam has a built-in mic make sure you have it set as default.
TEST SPEAKERS
Right click on speaker icon in System Tray
Select "Playback Devices"
The speaker icon ( checked as working) and volume level graph should be visible
Right click on the volume graph and click "Test"
Left hand speaker followed by right hand should be heard

SET DEFAULT & TEST MIC
Click "Recording" tab.
To set required (webcam) mic as default - click on mic font to highlight and click "Properties"
In lower dropbox display "Use this device enable" click OK
Click "Levels" tab and ramp up sliders ( adjust to suit later)
Return to "Recording". Test mic by speaking and make sure volume graph displays

Restart computer

Configure to your chat program

Make sure your mic is set as default - especially if you have a laptop that already has a built in mic
TEST SPEAKERS
Right click on speaker icon in System Tray
Select "Playback Devices"
The speaker icon ( checked as working) and volume level graph should be visible
Right click on the volume graph and click "Test"
Left hand speakers followed by right hand should be heard

SET DEFAULT & TEST MIC
Click "Recording" tab.
To set required (webcam) mic as default - click on mic font to highlight and click "Properties"
In lower dropbox display "Use this device enable" click OK
Click "Levels" tab and ramp up sliders ( adjust to suit later)
Return to "Recording". Test mic by speaking and make sure volume graph displays

Restart computer

You may have to tell your computer what mic you wish to use ( make it default )
Typical audio set up

Laptop/mic x webcam/mic x Vista x Yahoo Messenger
If you have an earlier OS, you may have to access your hardware from the Control Panel.

TEST SPEAKERS
Right click on speaker icon in System Tray
Select "Playback Devices"
The speaker icon ( checked as working) and volume level graph should be visible
Right click on the volume graph and click "Test"
Left hand speakers followed by right hand should be heard

SET DEFAULT & TEST MIC
Click "Recording" tab.
To set required (webcam) mic as default - click on mic font to highlight and click "Properties"
In lower dropbox display "Use this device enable" click OK
Click "Levels" tab and ramp up sliders ( adjust to suit later)
Return to "Recording". Test mic by speaking and make sure volume graph displays

Restart computer

CONFIGURE TO MESSENGER

Open messenger - on top toolbar. Left click ( > ) Messenger > Preferences > Connection - Select 'no proxies' or 'connect direct to the internet' depending on the version of messenger you have
make sure your internet connection and country are correct in the lower two boxes > Apply > OK

Click ( > ) Messenger > Preferences > Calling and Audio
Set mic and speakers to show Default equipment
Run call up assistant to test audio equipment > Apply > OK

Try setting up your audio from the beginning - I dont know your computer set up ( desk top with add-on speakers or laptop with built in speakers or your OS ) So I will give you a general example using :-
Laptop with built in mic x webcam with built in mic x Vista OS x Yahoo Messenger

If you have an earlier OS, you may have to access some of your hardware ( speakers and mic ) from the Control Panel.

TEST SPEAKERS
Right click on speaker icon in System Tray
Select "Playback Devices"
The speaker icon ( checked as working) and volume level graph should be visible
Right click on the volume graph and click "Test"
Left hand speakers followed by right hand should be heard

SET DEFAULT & TEST MIC
Click "Recording" tab.
To set required mic as default - click on mic font to highlight and click "Properties"
In lower dropbox display "Use this device enable" click OK
Click "Levels" tab and ramp up sliders ( adjust to suit later)
Return to "Recording". Test mic by speaking and make sure volume graph displays
Restart computer
CONFIGURE TO MESSENGER

Open messenger - on top toolbar. Left click ( > ) Messenger > Preferences > Connection - Select 'no proxies' or 'connect direct to the internet' depending on the version of messenger you have
make sure your internet connection and country are correct in the lower two boxes > Apply > OK

Click ( > ) Messenger > Preferences > Calling and Audio
Set mic and speakers to show Default equipment
Run call up assistant to test audio equipment > Apply > OK