After you select your teams and the game is loading you have a chance to enter a code here are some codes...
press B (first position), Y (second position) and X (third position) the required amount of times to unlock the corresponding cheat:

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