How could use vivtar 500mm in D40x. the seller asked me to put the camera in Manaul mode and set off to the shutter priorty to off. I could not find how to set the shutter priority to off in the setting.

A 500 mm mirror lens is a fully manual lens (manual focus no aperture change) using a T mount adapter to your specific camera. The T mount costs some $15. It depends on your camera how to turn off the auto focus and go to aperture priority mode. The aperture on a mirror lens is set, usually at F8, so the shutter is the only item that can be adjusted to get a correct exposure. A digital camera (set in adjustable ASA setting) can change it's ASA to help with getting a correct exposure. Manual for digital camera are usually on the companies' web site.