Cannot set up microphone on my acer aspire one d255

You don't specify what type of microphone you are using.
The following instructions apply to a microphone plugged into the normal mic/line in (red) jak-plug socket on most computers.
If you are using a USB microphone the process might be slightly different because the system has to actually recognise the microphone first. Most are plug and play so will automatically be detected after a few seconds but some do need drivers installing from the mic manufacturers disk
To use any microphone with your computer you will need to enable it in control panel. The following instructions are for Windows XP but are broadly similar for Vista and Win7
Click START, CONTROL PANEL, SOUNDS & AUDIO DEVICES
In the Sounds & Audio Devices Properties window locate the SOUND PLAYBACK box and click the VOLUME button. Make sure that there is not a tick (check-mark) in the MUTE box against microphone in the list of devices and that the volume slider is at the top of the scale.
Also make sure that the MUTE ALL box isn't checked before closing the window.
Now click the VOLUME button in the SOUND RECORDING BOX. Here you need to SELECT microphone as an input source for recording (as opposed to un-checking mute in the playback controls).
NOTE if microphone is not listed in the 2 sets of volume controls click OPTIONS, PROPERTIES on the tool-bar and place a check against microphone in the list entitled 'show the following volume controls' and then click OK.
That should be it.