You can't get the hovercraft from inside. Unlock the gear lock and go outside. Climb on the windmill arms and turn them counter-clockwise. This opens the roof dome.
The hovercraft runs on manure.
The hover board (hovercraft) runs on manure.
The fuel rod powers the saucer Excalibur. The hovercraft runs on manure.
The manure is used to power the hovercraft hidden in the attic of the mill. (see related question)
You have to power it up with manure which you have to get from the farmer near the secret hatch to Mordred's Hideout
The farmer you meet near the hay bales will give you a bag. (just one)
In Astro Knights Island on Poptropica, the fuel for the hovercraft can be found in the fuel canisters located next to the dock where you first arrive on the island. You need to collect the fuel canister and use it to power the hovercraft to cross the toxic swamp.
You first get the manure then you make the mill turn right then entert the top and go to your bag and hit use on the manure
In the field between the castle and the mill. You only get one bag, and it is used to power the hovercraft.
You use it to power the hover board (hovercraft) in the attic of the windmill. (see related question)
Go to the mud and you'll find it there. As soon as you fuel it, it stays right there in the swamp.
Once you add the manure to the hovercraft, it automatically goes to the point where it is used : in the mud swamp to the right of the mill.