I believe that it is in the map pack, but if not, then it is an online only map. Some maps on call of duty can only be played across xbox live, so you would have to just play in matchmaking til you found it.
You have to buy the map pack which contains Chinatown, Broadcast, Creek, and Killhouse. These maps are only playable online. Personally I bought a GOTY addition of Call Of Duty: MW and it came with a little leaflet with a code to unlock them for free.