At the option screen press LEFT LEFT RIGHT RIGHT LEFT RIGHT LEFT on the D-pad/lightgun. If entered correctly you will hear a sound. Once you play one of the game modes you will see 0000000 in the top left corner of the screen.
Enter the options screen and select "House Of The Dead 3".
You can now increase the credits past nine until "Free Play" appears. Alternately, play well and get better than an "A" rank one or more times during the game at the point where you lose all your credits. This works best in one player mode.