I am not MMM, but I can answer that.
In Forest of Illusion 2, near the end, the is a spot with a wall in a tunnel. you can go through this wall. there is a key, and this leads to Blue Swittch Palace. Sorry if that was confusing, but it's hard to explain.
*My real signature went on strike, said something about "unfair working conditions". So, this is a temporary replacement sig, until I can work things out with the real sig.