Go into the Museum and from the info booth go right and up the stairs. Now turn to the left and you will see a room next to the game rooms called the "XRoom". Go into this room and hold L1 L2 R1 R2 Up then press Triangle. To the right of the dark room will be a woman with green clothes. Stand in front of her and press X. She will disappear and you'll be left with the pedestal in the middle of the room and the words "Assault Plus" flashing above it. Access the pedestal and the arcade game will be the new Assault Plus game.