Extra Fuel
For 25 fuel units, hold A + B, then press Select, Left, Right when the sub course is displayed.
Extra Missiles
For 25 extra missiles, hold A + B, then press Up, Down when the sub course is displayed.
Extra Ships
For 25 submarines, hold A + B, then press Select, Up, Down when the sub course is displayed.
Level Select
At the main screen (where you choose One Player or Two) press B, SELECT, LEFT, RIGHT then press START.
Extra Fuel...