You need the Library Slip before entering the library. Go to the fountain, find a gold coin, and pay your way into the Mordred Museum. Once you have the Library Slip, you can access the hidden dungeon below the library. There are also two reference books (not required) that explain things about the Kingdom.
The library is in the Castle, to the right on the first floor. The three areas of the kingdom are the Village, the Castle, and the Mill.
in the castle
This is the first item on the island. Go to the Mordred Musseum and spend it. Get the library slip and go to the castle library.
The dungeon is in the castle library.
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
Mordred had often been going to the same location in the castle library (castle, first floor right).
There is a hidden stairway to the dungeon in the castle library, and a hidden opening under the fountain.
if you mean in astro knights, the mcm is in the library in the castle
It is on tp of the castle roof in astro knight island
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.
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.