Weird... Well, that's what should work. Anyway, you can try to download the program from other places, just do a google search for "iChat AV download". (They dropped the "AV" part for Tiger, so you should be able to find a link for 2.1, the last version for Panther, with this).
Otherwise, you could try to use the application Pacifist to try to install iChat from the installation discs that came with your iBook. Here's a link
http://www.versiontracker.com/dyn/moreinfo/macosx/12743