First, you must get to the Secret Options Screen. To do this, go into Options and highlight any of the four boxes. Then hold Up + L1 + L2 + R1 + R2 until you hear a "Boom." The screen will also shake. Then the "?" is selectable. one of the options in the "?" screen is for One Button Fatalities. Here are the button configurations to perform these fatalities:
At the Character Select Screen, highlight Rayden, Jax, Kano, or Kung Lao, and press SELECT. The selected character will explode and change into the classic version.
Move to Sonya Blade at the character select screen and press Up and START. You will hear the Continue "boom" sound. After picking your character, you should be able to pick the stage.