You need to complete the campaign in any difficulty to unlock the map "Five" or you can stand up in the main menu by pressing 'space' key and go to the computer behind the chair then hit the 'f' key and type '3arc unlock' and press 'enter' to unlock all maps..
do as he says... but after putting "3arc unlock" press "doa" into the pc and enter... it will give you the zombie arcade game... lots of fun