Ok I Have the same problem as you sometimes this may or may not work but keep on pressing F1 until you hear the Activated and make sure volume is up and this is at the main menu. Here is another option go ahead and start game I mean a mission then pause game and again do the same as i have said before.Lastly are you using a cheathappens maid trainer.HOPE THIS HELPS
So I was having the same problem (Windows 7 64bit). After running in compatibility mode as Windows XP SP3, I only had to spam the CRAP out of the F1 key from the main menu till it finally said "Activated!". Thanks for the tip