This would be on the second level of the Bat Cave. There is a big computer in the middle of the screen above where the outfit changer is where you can enter the codes.

Password - Effect

ZAQ637 - Unlocks Alfred in Free Play
DPP967 - Unlocks Commissioner Gordon in Free Play
XVK541 - Unlocks Free Girl in Free Play
RDT637 - Unlocks Harley Quinn's Hammer Truck
XWP645 - Unlocks more Batarang Targets
BJH782 - Unlocks Penguin Henchman in Free Play
NJL412 - Unlocks Yeti in Free Play