Cheat Codes/Map Select:
To load any map of your choice, go to the Steam control panel, Right Click “Left 4 Dead 2” and under the “My Games” tab select “Properties. In the General tab, select “Set Launch Options”, enter “-console” in the field, and accept it. Start the game, and the console window will appear at the main menu. Enter “bind p toggleconsole” as a console command. Then, type one of the following “map [map name]” codes and press [Enter] to load the corresponding map. The same entries can also be used with the “changelevel [map name]” or “changelevel [map name]” codes during the game.

Alternately, at the main menu, select “Options”