How do I access the boost pad on the wall in Sherbet Land?
Directly after the starting line, there is a giant crack in the ice. If you were to go around it, you would find yourself looking down the tunnel in your screenshot, with the boost pad visible to the left.
If you were to drop down into the giant crack, you would have complete the next part of the race under water. If you follow the left path, you will eventually come out at this boost pad.