By default, it should be off when you plug your mouse in. Here are instructions on turning it on. Check this preference file and see if it is set at "0." The file may be corrupt.
1. First, you need to modify a "hidden" preference file. To show hidden files, use a free utility like OnyX.
2. Once you can see hidden files, navigate to your User preference folder and look for .GlobalPreferences.plist. Note the file has a "." in front of it to signify it's hidden.
HD > User > Library > Preferences >
.GlobalPreferences.plist
3. Open that preference file with a text editor and find "com.apple.mouse.ignoreTrackpadIfMousePresent" and change the valuse from 1 to 0.
4. Log out and log back in. You should be all set to use the trackpad again.
Dave M.
MacOSG Founder/Ambassador An Apple User Group iTunes: MacOSG Podcast
Creator of 'Mac611 - Mobile Mac Support' (designed exclusively for an iPhone/iPod touch)