At the screen where you select "Exhibition," "Roster," and other options, press L1, R1, L2, R2. This takes you to a screen titled "Easter Egg" with two nwe options""Add Entry" and "Remove Entry." Enter the "Add Entry" option and enter any of the following (upper and lower case does matter when entering these codes):

Cruel CPU
Fast Clock
Jack T
Loose Cover
Slow Clock
Slow CPU