At the mode select, highlight 1P Play and press Right on the directional pad four times.

Highlight Tournament and press Right on the directional pad one time.

Highlight Course Edit, and press Left on the directional pad twice.

Highlight Option and press Left on the directional pad six times.

Now highlight 1P Play and press X, you should hear the crowd cheer to confirm the code, you'll have 7 lives instead of just three.