Simply load up the map 'Tiger Temple' and after you exit the first hallway (halfway through the lap) you will notice a big door on the left-hand side. You will need to use an item against it to open. The following will suffice:
- Bomb (rolled forward)
- Beaker/Potion (thrown forward)
- Missle (use as close as possible so that it doesn't attempt to track another racer)
I created the following video to help:
Hope this helps.