Go to options, then go to the cheat menu. select one of the cheat options and then just type in the code. You just have to move your cursor over a cheat option to select it. If the code is successful you will hear a sound and there will be a "on" icon appear next to the code. When done make sure you press "X" rather than"O" or else all the cheats will be turned off.

Password - Effect

L1, <-, ->, square, triangle, R1 always invisible
L1, sqaure, triangle, ->, triangle, R1 Insta gib
->, <-, ->, <-, ->, <- max players
Triangle, L1, square, square, ->, <- Mortar only
start, ->, <-, select, ->, sqaure no auto heal
sqaure, ->, triangle, ->, R1, L1 pick up double time
->, ->, <-. square, R1, L1 Team special recharge
<-, R1, L1, triangle, ->, square unlimited ammo