From the main menu go to "Options", then "Player Profile", then finally "Enter Cheat"

$moneybags$	Gets you 1500 Credits.
All Access	Unlock everything except the Fury bike.
Billboards	Unlocks all Music Videos.
Duds		Unlocks all Rider Gear.
Dubs		Unlocks Rims.

Easy ATVs
Beat all the courses in the training mode and you will get an average of Gold, Silver, or Bronze. Each average gets you a prize, and they are as follows:

Gold = 3 Talon ATVs
Silver = 2 Talon ATVs
Bronze = 1 Talon ATV.