At the beginning, the game opens up with a menu (single player, multi player, etc.). Go to "Options", choose "Cheats." Enter the cheat, and press enter.
Cheat Code - Granted Wish
BIGHEAD - Big Head Mode
MR HAPPY - No Dressage Failures
Phar Lap
When entering the Main Menu of Equestriad 2001, click on "Options" then on "Cheats." Press "Enter Cheat." Type in Phar Lap. You'll get unlimited energy on the cross-country courses.