"McM" is the shelf letter on the non-fiction (lower) aisle in the castle library (first floor right side). When you have the Library Slip, you can click the scroll on the "McM" shelf, and a stairway to the castle dungeon will open.
(see related question)
if you mean in astro knights, the mcm is in the library in the castle
it is McM
under the library, you click on the McM label and stairs appear
McM
in the castle library click mcm in the non-fiction section to get into the dungeon
Go to the library and then there will be a sign that says McM. Click on it and you're in!
McM is the shelf label for the lower aisle of the castle library. If you have the Library Slip, you can press this shelf to open the dungeon passageway.
The library SLIP is marked "Non-fiction McM", which is the lower aisle in the library. Press the spot on the shelf marked McM.
The McM is on the bottom floor in the library. You can click on the mcm if you have the library card. The library card is in Mordred's house museum.
you click on the paper under mcm
The dungeon appears if you click McM, after you got the library slip from Mordred's House/Museum. You can get the coin from the fountain, where it is shining.
Once you have the Library slip, you can press the scroll McM on the lower aisle in the castle library, which will open the dungeon stairway.