When trying to join an Multiplayer game you get an "Error, Key code in use" message.
IF YOU HAVE PREVIOUSLY JOINED A MULTIPLAYER GAME SUCCESSFULLY:
The 'CD key in use' error message displays when your serial number is still bound to a previously played server. Your router and the server host will bind your key and cause conflicts when trying to join another game.
Be sure that your computer is not behind a software or hardware firewall of any kind. This includes firewalls built into routers or other equipment. Aspyr does not support the use of our games from behind a firewall. If it is not possible to disable the firewall you will need to open the ports for Call of Duty 2 and Call of Duty 4: Modern Warfare. Call of Duty 4 and 2 transmit and receive network traffic on port 28960 (UDP/TCP). The exact details on opening a port is specific to your firewall. For more information check with the firewall documentation or manufacturer.
IF YOU HAVE NOT PREVIOUSLY JOINED A MULTIPLAYER GAME SUCCESSFULLY (mostly occurs with the App Store releases of Call of Duty 2 and 4):
Please delete the com.aspyr.callofduty4.plist/com.aspyr.callofduty2.plist file from User/Library/Preferences to reset your cd key. >p>Be advised that if you delete this file, it WILL RESET STATS AND RANKINGS, so we advise that you only do this if you do not mind having these reset.