If you don´t have inet at home,your only choice is to put the rars on your stick and extract them.
Which extractor you use is your choice (7zip or Winrar are much the same).
Also the "How to" is the same:
1. download the track in rar-form
2. extract it with an extractor of your choice (7zip or Winrar) in a folder of your choice
3. Copy and paste it to your rfactor main folder
For example the track "a1-Ring" from this site
after you downloaded it and after extracting you get a folder called "gamedata". This folder you only copy to your rfactor main folder (if asked to overwrite type "yes").
After that you should be able to use the track in game.