I'm a newbie too, not even built a system, BUT I have MediaCar installed on my laptop, sop I know a little bit.
1. To have the skin changed, find the skin files and put all of them in a separate directory below C:Program Files\MediaCar\SKINS - then you will be able to choose this from the Skin menu (see how the DEAFULT skin is in the above mentioned directory.
If you have downloaded som skins in zip-files, the problem is usually that unzipping them to the new directory below the C:\Program Files\MediaCar\SKINS directory, creates subdirectories below this directory again - for instance the Alpine Blue skin files created this structure for me:
where I had made the "AlpineBlue" directory myself. So unzipping it directly to the SKINS directory would have done the whole thing automatically!
2. If you want to see the cover of the movie to the left, put a JPG file with the picture directly in the directory of the movie (maybe you have to have each movie in a separate directory or how would the application know which one to choose if not?
Your last question is a good one - I thought you could set this up when you run MediaCar for the first time when the Setting window appear, but how do you invike this later???