Go to options, then select cheats. Now type in any of the following codes in lowercase:

Code - Result:

iwantitall - Activate all of the cheat codes
fleximan - Extends your Tricks Slot to 31
garageking - Unlock bike parts
johnnye - Unlock character costumes
universe - Unlock levels and events
seeall - Unlock posters
dudemaster - Unlock riders and all bikes
hearall - Unlock all songs
watchall - Unlock all videos