Complete the requirements to unlock the extra stages.

Stage	How to unlock
Arctic #2	Obtain 132 stars, and play with 3 Facebook friends.
Desert #2	Obtain 108 stars, and play with 1 Facebook friend.
Dunes #2	Obtain 156 stars, and have the Ninja Bike unlocked.
Hills #2	Obtain 180 stars, and have the Silver Bike unlocked.