First, select the Shack piece that sais Wet Beavers on it. Then raise some land behind the building like show in the pics. Now simply go to test play and jump into the crack of the back of the building and the raised land. This can be annoying sometimes, but all you have to do is ollie and spin a few times and try to get wedged into the building. Once you are far enough, you will be able to skate around inside the shop.
Click on a thumbnail image above to view a larger sized image.