Press the console key ` and type "cht_bEnableCheats = 3" to enable the developer cheats. Note that console commands are case sensitive. You can access the cheats through the menu (Press ESC then press F3) or by typing them into the console. (Type "cht" into the console and press TAB for a list of cheats.) Doing this however will invalidate your achievements and leaderboard score gained in that session. Type "cht_bEnableCheats = 0" in the console to disable the cheats for the next session.