You have to get the moldy cheese from the library's dungeon entrance. Press 'Use' on the cheese in the Princess's room.
(To get the cheese, take the gold coin from the fountain and pay your admission into the Mordred museum. Get the library slip from the stack of books. Then when you go to the castle library, click the McM shelf on the lower aisle to enter the dungeon.)
Feed it the mechanical mouse (from the Maid's room) and the owl will follow you.
release it to the owl. then the owl will follow you
You scare the owl outside. follow it and release the mouse. it will know follow you
You have to feed him the mouse. Merlin the owl will follow you into the passage to the dungeon cell, and you can click on him and then the fuel rod and he will get it for you.
give him the robot mouse from the cell
Follow the owl out of the lab and feed him the mouse (from the Maid's room). He will follow you. Click the owl, then the fuel rod and he will retrieve it for you.
It is a mechanical mouse. (see related question)
Click on his card in your items bag.
The mouse will get surprised, run around, then run back to you.
Feed the mechanical mouse to Merlin and he will follow you to the dungeon. Click on the owl, then click on the fuel, and he will retrieve it for you.
Go outside and feed him the robot mouse, and he will follow you into the dungeon. Click the owl and then the fuel rod and he will retrieve it for you.
Follow him out of the lair and feed him the mechanical mouse. Then he will go with you sometimes. To retrieve an object, click Merlin then click on it.