What does a key code in use means

I only know how to fix it for Modern Warfare:
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.