Instructions
Press the ESC key on your keyboard while in-game to open the main menu and select "Options." Click on "Advanced" and select "Enable developer console." Click on "OK" to return to the game.
Press the "~" key to open the developer console and type "sv_cheats 1". Press Enter.
Type "mp_allowmonsters 1" and press "Enter."
Enter "developer 1". Press the Enter key.
Type "give monster_name" and press Enter. Replace "name" with the name of the monster you wish to spawn. For example, if you wish to spawn zombies, type "give monster_zombie".