you cant i have that same problem too
next to a bookshelf.
IS IN DA LIBRARY... SOMWEAR.... and i dongt
The dungeon is in the castle library.
under the library, you click on the McM label and stairs appear
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.
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
Use the cheese from the dungeon (under the library in the castle). (*To get to the dungeon, see the related question.)
There is a hidden stairway to the dungeon in the castle library, and a hidden opening under the fountain.
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 only things you do in the castle dungeon (under the library) are pick up the cheese and throw the switch. (see related question)
One of the robots was captured and is in the dungeon.