Start a Game, can be Single Player or Multiplayer, once in, pick any side you want and begin the game, start fragging as many Tech-Com's/Terminators as you can and have the high score in the game at the moment. As soon as you die and go to the Respawn screen, select Tech-Com and switch thru till you see The Terminator (aka Arnold in his T3 Outfit), now you can play as Arnold on the Tech-Com side.