Legends mode:
Unlock Challenges 62 through 66. Challenge 67 will be legends mode.

Unlocking characters easily:
Start stunt mode and win the race under the pro difficulty setting. Burn out at the 
start until the counter reaches about 3,000 and do not go off the track (or you will 
get the clean section). Do this for about eight tracks to unlock all the characters. 
Note: You will need a bike with at least all stats at about halfway on each bar. 

 
 Game Shark Codez: 
 
 Master Code (Must Be On):
 0E3C7DF2 1853E59E
 EE938E8E BCD1E162
 
 All Challenges Complete:
 BEAF9F7A BCBB9B84
 F0FCFCF8 BCA99B83