Hey there-
Make sure your computer has iTunes 7.3 installed.
Click your iTunes menu, then About iTunes.
If not, download it here-
http://www.apple.com/itunes/download/
If you do have it, go into your Applications folder and uninstall iTunes (drag it to the trash). Restart the Mac and download/install iTunes again. Not sure why, but I've seen this with a lot of Macs... in some cases it was caused by iTunes 7.3 being installed before OS X 10.4.10.
-CG