How do I remove the film door on my Nikon N90

http://uscamera.com/1b999-517.htm

That's for the back cover for the Nikon N90/N90S, which is what the F90/F90X are known as in North America.

We will assume you know how to load the film to the red mark. If the film is not advancing a couple of things could be happening. When you close the back door it is not engaging the film to hold it against the take-up spool. If so then that needs to be fixed. The other thing to check is the left side where the film cartridge goes in, does it turn. If not it is binding the film cartridge. Nikon Film 1-800-645-6687

Nikon doesn't supply the film door any more. You can buy a repair kit from USCamera.com to replace the latch on the film door

Can it be repaired? It needs a new latch piece. Maybe a new door, if its all one piece.
I do not own one so not sure about that model. Look at the door hinge. If a small lever sticks out of the hinge, it may be easily removed and replaced with a new door. otherwise you need to remove the bottom plate of the camera to replace the door. Be very careful doing so as things could pop out when you open it.
You could also send it in to a repair shop, where they will also clean, lubricate and adjust it before returning.

Is it worth it? Only you can answer that. You might be able to pickup a used one on Ebay, so you have a source of parts.
If you have no reason to keep that particular camera, replace it with a newer Nikon, so you can keep using your flash and lenses.

http://www.butkus.org/chinon/nikon.htm

Press down and slide one way or the other. It's generally held by two pins that slide into hole slots and lock in place

Sometimes removing the batteries for a day may correct the problem temporarily. But the real problem is really in the lens mechanism. "Lens error"can only be repaired by Nikon or an authorized repairman. I sent mine back in Dec. 2007. It was repaired free of free of charge under the warranty. It was fixed and returned very fast.
The same problem just reoccurred and I sent it back to Nikon this very morning. They told me this is not a problem with this model, but I beg to differ. My internet search revealed that this apparently is a problem with many Nikon lens mechanisms.

"End" can be a message for "film over" or for a film jam in the cam. Did you check for this already?

Along the hinge side near the top, a small screw sticks out from the plastic end of the film door. (opened). Push the screw down with your finger to release on edge of the door and lift it free of the camera

You have to replace the whole door. There is one on eBay, but you won't like the price. Find it here: eBay