Here's what the help file says:
"If your computer is connected to any other computers over a local network you can share the music in your library and playlists with up to five of those computers (the computers need to be in the same subnet as your computer, and Windows computers need to be running iTunes 4.1 or later and Macintosh computers need to be running Mac OS X version 10.2.4 and iTunes 4.0.1 or later). Sharing is intended for personal use only."
Could that be your problem? I'm not on a network at all so I can't share files. Dammit.