You are trying to jump ahead of the game. You need the Library Slip from the Mordred Museum, which shows that Mordred was getting books from that section.
Get the gold coin from the fountain, pay your way into the museum, and find the Library Slip in the stack of books. Now when you go to the castle library, you can click on the non-fiction (lower aisle) where it says McM.
Go to the Mordred Museum and get the library slip. There is a gold coin in the fountain to pay the fee. Go to the castle library (first floor right) and see the lower-aisle shelf marked McM. Press it and the secret passage to the dungeon opens.
(There are only two things to do : pick up the cheese and throw the switch. You cannot open the cell. See the related question below.)
in the castle library click mcm in the non-fiction section to get into the dungeon
There are no plants to click on. You click on the lower shelf in the castle library (McM) and click the "planets" when you arrive at them. On the Jungle Planet, the elevator plants work automatically when you jump onto them.
Use it to pay your admission to the Mordred Museum. Click on the stack of books and you will find the Library Slip. Once you have it, you can click the scroll on the shelf in the castle library, on the lower aisle.
depending on where ur at in the stage the underground place is in the library, click on the little sign under the book shelf with the "McM"
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.
Click on the stack of books next to the man and get the Library Slip. You can use this in the castle library, on the lower aisle where the shelf is marked M-Mc.
You will have to use the gold coin from the fountain to get the Library Slip from the Mordred Museum. Once you have it, you can click the McM shelf in the castle library and open the secret stairway to the dungeon.
"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)
The shelf marked McM in the castle library opens the hidden stairway to the castle dungeon. You can only click it if you have the Library Slip from the Mordred Museum. Get the gold coin from the fountain to pay your admission. The slip is in a stack of books. When you have the Library Slip, you can click on the McM shelf in the lower aisle of the castle library, to open the secret stairway to the dungeon.
The rolled up paper is on the shelf in "Non-Fiction McM". This is written on the library slip found at the Mordred Museum. (Get the shiny gold coin from the fountain and pay your admission.)
Go to the Jungle Planet, and jump down from the platform and run across the ground and it will be on a little shelf/platform thing in a spotlight.
In the lower aisle of the castle library, in the Non-Fiction section. You cannot use the secret passageway until you have the Library Slip from books at the Mordred Museum. Get the gold coin from the fountain and find the slip in the museum.