Open and quit iTunes once in Leopard. That should guarantee that it will create an empty iTunes folder (not sure if it's there by default). Now go to your users folder / music. In the music folder there should be a folder called "iTunes." You can replace this folder with your "iTunes" folder that you copied to your thumb drive. If the folder you saved was called "iTunes Music" (if you didn't copy the whole iTunes directory) then you should go inside of the "iTunes" folder in your music folder, and replace the "iTunes Music" folder instead.