Getting the Fuel Rod
Even if you see the fuel rod, you may not be able to successfully grab it if you don't know what you're doing. At the end of Mordred's Secret Hideout, you will see a robot blow up and the fuel rod fly behind the bars of a jail cell. You cannot grab this fuel rod with your avatar alone. Instead, hover your mouse over the owl in the room, click on it and ask it to retrieve the fuel rod for you. Once the owl has the fuel rod, click on your avatar to direct the owl to give you the fuel rod.
Unearthing the Spaceship
Return to the windmill, which is located to the right of the entrance to Mordred's Secret Hideout. If you did not previously enter the windmill and unearth the spaceship, do so now. First, grab the rope hanging from the wheel to the right of the windmill entrance. Inside of the windmill, speak with the pale girl. Spin the large fan by jumping atop the sign and then jumping on the motionless blades. Eventually, the windmill roof will open. Jump through the roof and see the spaceship.
Releasing the Spaceship
To enter the spaceship, move the spaceship outside of the windmill and into the mud outside. To do so, open your inventory and select the bag of manure that you received from the guard earlier in the adventure at the Olde Rumour Mille. Drag the manure to the spaceship to get it running.
Using the Fuel Rod
You will need to use the fuel rod to repair the spaceship. Once inside, look for the broken fuel rod that's spurting green goo in the lower right side of the screen. Place your green fuel rod in the place of the broken fuel rod and it will fix itself automatically. Enter the coordinates 56-52 into the central control panel to start up the spaceship.