iTunesMouseRemote is a program which allows you to control iTunes using your mouse as if it were a remote control. Clicking your middle mouse button (or clicking the scroll wheel if your mouse has one) will play/pause, clicking your left button will go to the previous track, and clicking the right button will go to the next track. Rolling the mouse wheel up and down will change the volume. Obviously, this program requires a mouse with at least three buttons.
Installing iTunesMouseRemote is pretty easy. Just run the installer program provided in the zip file. An application called "iTunesMouseRemote.app" will be copied into your /Library/iTunes/Scripts/ folder. iTunes itself is left untouched, so if you ever have problems with iTunesMouseRemote, uninstalling it is as easy as deleting the file mentioned above.
If you want to manually install iTunesMouseRemote, it is quite possible to do that, though it takes a little bit more time. Just right-click on the installer and pick "Show Package Contents...", then open the Contents/Resources folder. Copy the iTunesMouseRemote.app file to your user /Library/iTunes/Scripts folder (you may have to create some of these folders first). Then, use Terminal.app to create a symbolic link with the follwing command:
ln -s ~/Library/iTunes/Scripts/iTunesMouseRemote.app ~/Library/iTunes/Scripts/iTMRProxy.scpt
This tricks iTunes into displaying the application in its Scripts menu.
I'm not sure which versions of iTunes this will work in, though it is guaranteed to work in iTunes 4 or later. It probably will work in earlier versions, though I can't verify that myself, since I don't have them. It does require a mouse with at least 3 buttons to work properly. It also requires OS X.
To uninstall iTunesMouseRemote, simply run the installer again and click "Uninstall." If you do not still have the installer, you can download it again to uninstall (even if the version on the site is newer than what you have). If you would like to manually uninstall iTunesMouseRemote, just delete the /Library/iTunes/Scripts/iTunesMouseRemote.app file and the /Library/iTunes/Scripts/iTMRProxy.scpt file.