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:

MR HAPPY = No Dressage Failures
BIGHEAD = Big Head Mode
PHAR LAP = Unlimited energy round the Cross Country
CHRISTMAS = SNOW You'll see!
GENIE IN A BOTTLE = Unlocks all tracks and horses
NEW YEARS EVE = Fireworks
TANTALUS BIG AIR = Ballistic Rider