Hey matuzz,
You needed to have preordered the game from a participating retailer listed here: Activision Support. Classic Ghosts appears as a multiplayer head option not an actual character, so if your code was valid it will be in the Create A Soldier Customization Screen, with a Male Character Model under the "Head" category, the mask should be near the bottom of the list. Can you confirm that you received a confirmation that your code was redeemed?
Thanks,
^AJ