There are two different codes to access the cheat menu for this game. One is for the One Player mode and the other is for Two Player mode.

One Player Cheat Menu:
Up, Up, Down, Down, Left, Right, Left, Right, A, B, Start.

Two Player Cheat Menu:
Up, Up, Down, Down, Left, Right, Left, Right, A, B, Select, Start.

You need to input the code extremely fast. If the demo screen comes up, you'll have to hit Start and start inputting the code from the beginning again.