1. You don't have to start a split screen game, just hit the split screen link and then back out to solo mode and it will work.

2. I have learned that this cheat works on all Zombie levels.

3. When you quit out of your game, you are taken to the split screen menu again as opposed to the solo menu option. The reason I point this out is that the possibility exists that the game is counting your game as a split screen game and not a solo game, in which case it will not record your score if you acheive a new record level (the game does not count split screen games in the Barracks). I have not been able to break any of my records to test this out so I cannot confirm this part is true, but please keep this in mind when using this cheat.