Like most things in life practice makes ease of use.
I think the Calibre ui is quite easy to use and usually requires only several clicks to accomplish most tasks.
However it does require some basic ui literacy/understanding that all ui's generally require.
Perhaps a simple flow chart on MSWord, etc. showing the basic functions would be a simple way for the occasional user.
I've taught friends and family how to use a variety of devices and interfaces by the practice/repeat method. Start with several basic functions and have them repeat it until it becomes second nature. Then move on to another function...
Most people feel they can't use a device or interface because they see it as a complex thing that is so fragile it will break or not function at the press of a wrong button but said person can create a wonderful meal, etc. from scratch or from a cook book, etc that is far more technical and complicated where doing something wrong may indeed screw the pooch...it is just practice and familiarity.