At last success, just tried the code again and it was accepted with no more invalid code messages. I guess the answer is to be persistent and just keep entering the code till it works. Good luck everyone hope it works out for you. Oh and no need to have the game first, I'm still waiting for mine to be delivered but that's a whole different topic lol :)
My code isn't working either. To top it all off, I am missing my hardened edition playercard codes and the weapon camo code unless I just went past them. I got everything but these and its tickin me off big time.