First of all, I'd like to comment that you didn't pay all that money for "NOTHING" just because the Forward Unto Dawn code didn't work, you still have all the physical stuff and armor codes and what not, but I agree, not getting what you paid for is very angering. Now, onto your issue.
The card with the code says "Once redeemed, follow the additional instructions to access Halo 4: Forward Unto Dawn on Halo Waypoint through your Xbox 360 and the web." Aside from that I can't help you, because my xbox just crapped out.