This key is located in the track called "Boulder Canyon". 
Get to the part of the track where you see a bell hanging 
from the ceiling. Power up your speed burst to level 3 
(get three blue balloons). The turn around and face the 
bell from the "wrong way" then hit the bell. The drawbridge 
in front of you should raise. Use your level 3 speedburst 
and speed up the raising drawbridge and get the key. 
(Make sure its a 2x burst, and be sure to jump right 
when you reach the top of the bridge!) 
Submitted by Bryan Meason, Drew Ressler and The Game Dr