With Call of Duty 4 Mac to Mac MP accessible only at this time, here's how you can join other Mac players in combat.
1. Go to: http://www.gametracker.com/search/cod4/
2. Select a server that specifically includes 'Mac' in its title.
3. In the COD 4 MP Options menu, set Enable Console to Yes.
4. Press the '~' key on your keyboard to display the console command line.
5. Type in connect (IP NUMBER FROM GAMETRACKER LIST) then press the ENTER KEY.
The process is demonstrated here: