Ive had this problem before with other games (sadly the only i ever fixed was Counter-Strike Source)

Im a mapper and i just got Left 4 Dead 2 from a friend a few days ago. I want to make a map for the game but in Source SDK, in that little drop down thing at teh bottom where you choose which game configuration to use, Left 4 Dead 2 isnt anywhere.

Does anybody know how to fix this? Please?