Unlock Cheat Menu:
At the Main menu press Up, Down, Left, Right, Down, Down, Right, Right, Square.
When the Core Design copyright message appears quickly press Up, Down, Left, Right, Down, Down, Right, Right, Triangle.
Begin a game and then quit by pressing Start + Select then at the Main title screen press Up, Up, Up, Down, Down, Down, Right, Right, Triangle. If the code has been entrered correctly you will hear a chime as confirmation.
Unlock 'Payback' Mission:
When you have entered the briefing area find the locker on the left side of the screen and open it. At the password screen enter 4L2CZCQKDAABACQDB and a mission to destroy the Crimson Blade training camp in the Masirah Mountains will start.