For anyone still confused or not able to get it to work online:
1. Download pbsetup for mac from here:
http://www.evenbalance.com/index.php?page=pbsetup.php
2. Install it in Applications.
3. Open it and click add game, select Call of Duty 4, then copy and past the path below into the path box.
/Applications/Call of Duty 4.app/Contents/Call of Duty 4 Data
4. Then click "file > check for updates".
To edit the permissions if they are grayed out, click the lock in the bottom right hand corner of the get info window. Change them to Read and Write on the contents folder and app, then on the multiplayer app and it's contents folder.
Just to be sure I added more paths on PunkBuster by adding the game again, here are the additional ones I added:
/Applications/Call of Duty 4.app/Contents/Call of Duty 4 Multiplayer.app/Contents
/Applications/Call of Duty 4.app/Contents
If It doesn't work now, then I guess you have to contact Aspyr, but since they made the port and apparently didn't test it (or bother to fix it), I'm not sure there's anything more you can do unless it's an unrelated problem you're experiencing.
Oh and to find servers make sure that the source is: Internet, not Favourites or LAN, then just hit Refresh and wait. If no servers come up it might be because of your internet connection or something else.