Double-click on Album Art Thingy to start it. You'll see a new icon in your status bar:
Click the icon and select Album Art Thingy to open the preferences panel. You can select whether to run Album Art Thingy at login.
Album Art Thingy runs in the background - you won't normally need to interact with it at all. When iTunes it playing, Album Art Thingy monitors the tracks you're listening. When it finds a track without album art it goes to the internet, retrieves the artwork, and saves it to iTunes for that track and any others from the same album.
If Album Art Thingy is running, click its icon in the status menu and choose Quit. Then follow the instructions for installation above.
If Album Art Thingy is running, click its icon in the status menu and choose Quit. Then drag Album Art Thingy from your Applications folder to the trash.