For the level code, enter the sequence at the mission select screen. For the other codes, you'll need to pause the game, and while at the pause menu hold down the Back button, Left Trigger, and Right Trigger before entering the password string.

Y, RB, Y, RB, X --> all levels
LB(x2), RB, X, RB, Y --> full life
Y(x2), X, RB, X, LB --> scott mitchell invincible
X(x2), Y, RB, Y, LB --> team invincible
RB(x2), LB, X, LB, Y --> unlimited ammo