You have to make sure you have Mordred's Library Book Slip before you touch McM. To get you need to get the gold coin by going through the top part of the rocketship shaped fountain. After you go to the Museum a.k.a Mordred's house. Give the guy the gold coin the go to your right. You will see some books and then click on them and I think the guy will give you the Library Slip. Then go to the castle then go to the library and touch McM. -Good Luck.
Use the code backwards then press on the sun. it opens the fountain.
You have to press launch!
Saturn star sun in that order
You press the moon, saturn, the star, then the sun
you go to the library and press on the piece of paper on the shelf.
The library SLIP is marked "Non-fiction McM", which is the lower aisle in the library. Press the spot on the shelf marked 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.
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.
It's downstairs. Press one of the stacked books with a paper on it, and it should get you the library card.
If you have the library slip, press the brick over where it says McM. Then, go down the steps.
You get the paper the girl gave you in the Mille and go to the fountain and type it in that order and then press the sun and go down.
In the castle library (first floor, right) there is a shelf marked McM on the lower aisle. Press it to open the secret stairway to the dungeon.