how do i uninstall an app in mountain lion?
MAMP has a specific uninstaller in the disk image you installed with. Mount that disk image and you should see the uninstaller. Go here for more information.

For the rest of your apps that don't have specific uninstallers, use AppZapper, which is a nice utility. There are a few alternatives (CleanMyMac has a built in function for this) http://www.appzapper.com/