Passwords brought to you by 
Unlock all Decay missions
Go to the Options, then Cheats. To enter the code you must have two controllers.
p2_R3, p1_L3, p1_up, p1_up, p2_down, p2_down, p1_left, p1_right, p2_left, p2_right, p1_X, p1_O, START
All Decay missions unlocked including bonus mission Xen Attacks