Halo 4 owners must register on a promotional website before applying their codes to the game, but even those who have done so are claiming their double XP rewards as missing in action. Microsoft has yet to respond to the Halo Waypoint forums in which the complaints originate.
We've contacted Microsoft to see if this is an acknowledged issue and, if so, what steps are being taken to rectify it. In the meantime, it may behoove you to hold back on the double XP and level up like the common people.
Update: A Microsoft spokesperson told Joystiq that the company is aware of the issue and has already resolved it. "Last week we were made aware that some users were not being properly rewarded Double XP, and we promptly addressed the situation. Double XP rewards are now working as intended. There are still a few users that we are currently working to credit their accounts with Double XP games that were lost."