How do you fix the "Invalid Code" problem when redeeming the 4 extra co-op black ops zombie maps?

I bought Black Ops: Hardened Edition, press the big X button on the xbox 360 controller, go to redeem code, put in the code, triple check to make sure its right, click enter, but it says it is an invalid code. What do I do to fix this?How do you fix the "Invalid Code" problem when redeeming the 4 extra co-op black ops zombie maps?
Call the customer service number on the back of the box. Make sure you have the receipt and they should tell you what to do.