Im using a4tech cam pk-635k i cant use in yahoo

Test your webcam here to make sure it broadcasts ok - Hit 'Allow' in the top window and check your cam in the left hand panel.

http://www.testmycam.com/

Having proved your cam is working ,you can configure it to Yahoo Messenger.
COMPATIBILITY WITH YAHOO MESSENGER
Use Internet Explorer to visit this page to check compatibility and automatically download the latest webcam control

http://help.yahoo.com/l/us/yahoo/messenger/messenger10/webcam/mstacamtest.html

CONFIGURE SOFTWARE
Its good practice to disconnect and re-connect your webcam before you start - if you have been using it elsewhere the Yahoo software may have difficulty in locating it. Mine, I just unplug it from the USB and plug it back in, if yours is integral with the computer you will have to re-start the computer, use a keyboard shortcut or disable / enable the webcam from the Device Manager.
Of all the chat programs, Yahoo is the most painful software to set up but if you follow these instructions you should be ok.

INTERNET CONNECTION
Webcam connected
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

TESTING
> Messenger > My webcam - the warning panel will launch, click 'OK' and your webcam will start. You should be rewarded at this stage with your happy smiling face.

SET WEBCAM DEFAULT
With webcam still running > Messenger > Preferences > Webcam > Camera Source - set your webcam as default using dropbox. > Camera Settings - adjust settings to suit. > Apply > OK
Take a note of any additional software, if you have multiple choice for Camera Source, you may wish to uninstall any redundant webcam software later to prevent conflicts

PERMISSION OPTIONS
> Messenger > Preferences > Webcam - make your selections ( who you will allow to view you) > Apply > OK

IN USE
When you are both on line and wish to show your webcam > Actions > 'Invite to view my webcam'. This will launch an invitation on your contacts screen when they 'Accept' your webcam broadcast will start- There can be considerable time lag ( 1 minute) before they see you, so don't be too impatient ( You can check the status on the bottom left of your webcam panel. You can 'drag and drop' the frame to a convenient location by grabbing the top border.

Your contacts may need to configure their connection the same as INTERNET CONNECTION above, (less the reference to webcam running) or they may get a "webcam not connected" message