Are you: Stepping on the mill 'wings' from the outside? Turning it counter-clockwise? Doing it long enough?
Go inside the cover and you will find a hover disk.
You turn the windmill counter clockwise a few times around and eventually it will open the roof top. Don't go inside yet, because you need to climb down from the mill and get a sack of manure from a farmer. Enter the roof of the mill. Use the manure to power the machine inside. It will fly out into the marsh the the right of the mill, and will take you to a space ship.
Jump up to the roof inside the mill and pull on the rope, the go outside the mill and jump on the blades to make them spin counter-clockwise.
Turn the windmill blades anticlockwise and the roof will open. They man to the left of the mill will give you manure if you talk to him. USE the manure when you're inside the top dome on the mill. It will be used to power the space board and it will fly down to the marsh below the mill to the right. Don't forget to collect the rope hanging on the mill wheel at the edge of the marsh!
you use it in the factor, on the roof
Go inside the mill, on the second floor, and hang from the rope that unlocks the mill gears. Then go outside and climb on the windmill arms to turn them counter-clockwise. This will open the roof dome. (see related question)
The Mill on Astro Knights Island? Go inside and hang from the short rope next to the gears, which unlocks them. Go back outside and climb on the windmill arms to turn them counter-clockwise. This will open the roof dome to the attic, where the hovercraft is stored. (It is powered by manure.)
You find the symbols on the roof of the ancient ruins. and just press them in order to make OPEN.
ON THE ROOF
they are in the the roof of the mill and on the farm near the mill
You either jump from roof to roof or use the camo.
Climb to the roof and you can jump up and grab the key. The basement door at the left of the house will be open.