There is a ton of information here about this subject. Do a search on Fresnel here and in Google and be prepared to spend some time reading.
To answer your questions in short form:
A Fresnel screen is a lens and brightens your viewing screen. You can just use the ground glass if you like. It will be darker, but many prefer a plain ground glass to a Fresnel due to other problems introduced by it.
The position of the Fresnel depends on the design of your camera. You need to understand how it works and how your camera is designed. If you have a factory-installed Fresnel, use it in the orientation it came in.
All this will become much clearer after you have done your homework.
Best,
Doremus
www.DoremusScudder.com