While the scrolling text loops in the main screen, press trigger once (space or button) really fast. If the game starts as usual, repeat trying to get a faster pressing. As a reference, you can look closely the char under the 'L' from "LEVEL" and you'll see a key that appears briefly; if the key stays, you've enter the cheat mode.

Press during the title screen at the cursor up key, followed by [Space], and you can select your starting field.