Go to System Preferences : Sharing
There, select the 'Firewall' tab.
You might want to do a quick test, turning the firewall off on either or both computers for a moment. If music sharing works, with the firewall(s) turned off, great.
Turn it(them) back on, see if the list contains an item called 'iTunes Music Sharing (3689)' and check the checkbox to open that port. If there is no such item, you can create it by hitting the 'New...' button.
J