It is only unlocked if the player either uses a cheat code, or beats the campaign on Hardened difficulty. Doing the former only temporarily unlocks it, while doing the latter unlocks it permanently. There are four maps in Survival.
Does COD MW2 have Zombies? Unfortunately, no there is no Zombies mode in Modern Warfare II, nor will there ever be. It wasn't ever on the table for the most recent title in the Call of Duty franchise, and other content was certainly put ahead of it in terms of inclusion.