Technically I suppose the head is wearing the mask but where you pick it is under the head option. Since you are using it, you must have just forgotten where you found it. I would have thought that it would have been a special like the Ghille suits but nope. They went a whole different direction with it.
but there was no code for the ghost uniform on my game,just for the free fall map..i bought my game on gamestop,in germany
I got mine from Gamestop in the USA and the code was printed on the receipt when I picked up my preorder. I don't know if Gamestop Germany had the same deal or not but if you still have your receipt you should take a look at it.