To activate these cheats, go to the match-up screen as the game is loading and press B (first position), Y (second position) and X (third position) the listed amount of times, then press the cotroller direction indicated to unlock the cheat:

Log Bat: 0, 0, 4, Up
Rubber Ball: 2, 4, 2, Up
Extra Time: 1, 2, 3, Up
No Fatigue: 3, 4, 3, Up
Rocket Park Stadium: 3, 2, 1, Up
Roman Coliseum Stadium: 3, 3, 3, Up
16 Softball: 2, 4, 2, Down
Unlimited Turbo: 4, 4, 4, Down
Tournament Mode: 1, 1, 1, Down
Mace Bat: 0, 0, 4, Left
Maximum Speed: 0, 0, 3, Left
Maximum Power: 0, 3, 0, Left
Maximum Batting: 3, 0, 0, Left
Small Heads: 2, 0, 0, Left
Wiffle Bat: 0, 0, 4, Right
Big Heads: 2, 0, 0, Right
Pinto Team: 2, 1, 0, Right
Eagle Team: 2, 1, 2, Right
Lion Team: 2, 2, 0, Right
Horse Team: 2, 1, 1, Right
Todd McFarlane Team: 2, 2, 2, Right
Terry Fitzgerald Team: 3, 3, 3, Right