The beginning screen before you start the actual game, scroll down until you see cheats. Then put your cheat codes in.

Password	Effect
V3GTHB	Aquaman
ZGCEAJ	Atrocitus
XZKLKQ	Bane
4LS32K	Batgirl
ZWQPJD	Batman (Planet X, Zur-En-Arrh)
YC3KZZ	Beast Boy
APEKBV	Blue Beetle
5SW59X	Deathstroke
4HRERD	Doctor Fate
NQ46RC	The Fierce Flame
FQ4ESE	Frankenstein
95U7BM	Giganta
9WYGLP	The Joker
J6ANCT	Kevin Smith
B5ABPQ	Lobo
S7GSDE	Music Meister
N9CZ7S	Nightwing
H2VB8Z	Plastic Man
TRQTPS	Red Hood
QDQ3YL	Superboy