You can't, normally. It was only because there was a group of us in some sort of uniform that had to change clothes backstage, and then enter to get to either the stage near the castle, or up by Small World, if the group is doing a parade.
Did you ever see that weird building off the 5(?) freeway that is all curvy and twisty and changes color from green to blue and purple depending on the angle you look at it? That's backstage at Disneyland. It's right behind Toon Town.