When entering the various cheats involving button combinations in GRAW the buttons must be entered very slowly, with a pause of a second or two between pressing buttons, and a second or two pause between holding down LT, RT, and Back, and starting to input the combination.

You will hear a beep if the cheat is entered correctly. If the cheat is not entered correctly, or entered to fast, you will just hear the regular "click" sound that happens when you just randomly press in a non-functional button at the pause menu.

Note: these cheats work both in single mission selects, and also campaign mode. Not sure about co-op, online, or multiplayer.

The cheat for "All Levels" must be entered from the mission select screen, and not from the pause menu in a mission.

The main thing to remember, is to enter the combinations slowly.