You'll need to create a new protected port for iTunes to share through. iTunes shares via port 3689. So, select "Other" and then type in 3689, and then title it. If it works, you should see something like this in your Firewall window:
1G iPod 5GB : lucy 4G iPod Photo 60GB : SoaringRabbit| iMac G4 800Mhz | 17" Aluminum Powerbook| OS 10.3 | LaCie 160 GB External Firewire HD - 70 GB of Music!