If you have arrived there because of the information on the Library Slip, click on the shelf (McM) and the stairway to the dungeon will open.
If you haven't gotten the Library Slip, you are wasting your time. Get the gold coin from the fountain, enter the museum, and find the Library Slip in the stack of books. Once you have it, the shelf in the library will open the hidden staircase.
if you mean in astro knights, the mcm is in the library in the castle
under the library, you click on the McM label and stairs appear
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.
Go to the library and then there will be a sign that says McM. Click on it and you're in!
in the castle library click mcm in the non-fiction section to get into the dungeon
it is McM
it does not say library card you have to click McM because that what it says on the library card again not library card
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.
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.
In the castle go to to the library (1st floor on right) find the little labels that say McM. Click it and stairs will appear.