How do you get to mcm on Astro Knights Island?

McM is the shelf label for the lower aisle of the castle library (non-fiction). If you have the Library Slip, you can press this shelf to open the dungeon passageway.

Use the gold coin from the fountain to pay your admission into the mordred Museum, and get the Library slip from the stack of books. In the castle library (first floor right), go to the lower aisle and press the shelf marked Mc-M (for Mordred).