Left, Right, A + B + C at the main menu.
A sound will confirm this part of the code. Then press:
B, A, C, A, Left, Left, Right, Left.
Press Start at the title screen. Press:
Up, Left, Right, Right, Up when the "Game Start" and "Config Mode" options appear.
The sound of an explosion will confirm correct code entry. Press Left or Right when "New York - Stage 1" appears to change the starting level.