Begin and save a game, then press RESET and wait for the American Sammy logo to and fully appear. Now press UP, DOWN, UP, DOWN, SELECT, START on controller 2 before the logo disappears. Select Continue and then press SELECT to bring up a sub-screen. Now, press START on Controller 2 and the word "Debug" will appear next to Status if you've done this correctly.
While playing, press SELECT to bring up a subscreen, then hit START on Controller 2.