Songs stored on an iPod are in a hidden folder and they don't have normal filenames, so you can't see them in Finder/Windows Explorer when you enable disk use. If you want to manually copy them, delete them from your iPod first by setting your iPod to manually update, then in iTunes, take all the songs off it. This is just so you'll have enough free space on it to copy them over. Next, drag your iTunes music folder to your iPod in Windows Explorer. Hook the iPod up to your PowerBook and drag your iTunes music folder from your iPod to your PowerBook's music folder.
Alternatively, you could just hook your iPod up to your PowerBook (without doing any of the above) and use Ollie's iPod Extractor to copy the songs from it. Ollies iPod Extractor is a little freeware application that copies songs and playlists from an iPods hidden music folder to your computer.