"To celebrate the launch of Call of DutyŽ: Black Ops II, Double XP is being activated this weekend, from November 16th through the 18th, for anyone that downloads the Nuketown 2025 map. This map is available to everyone that pre-ordered. Bring the action back to this classic map, re-imagined as a 60's depiction of the future."
I think it gets added to your score after the match is over. I thought I wasn't getting it either but noticed it seemed to be a higher match bonus. I was playing on Nuketown last night for a while and almost levelled each match (now level 40, gonna check out the SCAR-H).