Well, sticking with you doesn't help you much if there are no answers.
Upon checking, I find this web page which documents exactly what you have said:
http://www.megamacs.com/v1/index.php?action=view&pid=3232699&D=1
The keypad first became available on Amazon in 2003, well after OS X was released. Everything I can find says it should work, so I don't have the answer. I'm sorry.
You have a PC, too, so I guess you aren't too put out about going that route, but I so much prefer OS X over Windows (having tried Windows twice) that I couldn't bear to switch, myself. Have you checked out the features in
Tiger? Very nice. And Leopard, due out next spring, promises to be better yet. With me, it's all about the OS, and even though I know it's harder to find peripherals that are compatible, it's worth it to put up with that, in my case, in favor of the Mac operating system.
I can't help but think that it's a problem within the keypad itself, since other devices work with the USB ports on the iBook.