If you're in the game and you hit Esc, and go into Options, you'll see 'Cheats' there... if you go in there, there is a cheat option for green level. But enabling it makes it so you can't save your game. Kind of lame, but it might help speed along the process in discovering how to add it to the trainer.
Actually you have that backwards, the cheat is called "No Green Alert" and it does just that, it stops you from hiding and you will ALWAYS be under assault. It is in fact the EXACT OPPOSITE of what we are looking for.
Also you must complete 25 killing sprees in Campaign mode for it to even unlock.