Same problem here. I've tried putting it in multiple times and it just keeps saying
The code you entered is not valid. Please try again. Please note that the code is in upper case to help identify Ls from 1s. For more information, contact Customer Support here.
The Nuketown 2025 map code is in the box with the disc, pre order copies of the game should say "includes bonus Mao" on the case. Some people have been given the wrong copy with no code in the box, check amongst the booklet/flyers to see if it's there. If its no-where to be found you may have to take your copy back to be exchanged for the pre order copy. Hope that helps.
Tweeted Activision and here's the link to their response to me https://twitter.com/atviassist/status/268447706459013120
Good news though I did keep trying and eventually it did work but I must have tried about 20-30 times