Sega Master System / Mark III / Game Gear
Turn on the console holding + and wait for the Sega logo to finish (you can skip it by pressing ).
Press x8, x8, on the title screen. Use and to select the level (displayed at the top of the screen).
See the main page for Hyokkori Hyoutan Jima - Hyoutan Jima no Daikoukai (ひょっこりひょうたん島 - ひょうたん島の大航海)