You can start by reading the readme included in most addon tracks or cars. Usually the downloaded files go to \Steam\SteamApps\<username>\race 07\Gamedata. But to be 100% you should read the readme first and if it still doesnt work post again and we will try to help.
What exactly didn't you find, the readme or the folders that I mentioned? The readme should be on most addon and if not there should be an instruction somewhere on the download page itself. If not just place the addon files in the folder that I mentioned and most of the time it should work just fine.
RACE 07 is the mothergame in which all tracks will work. GTR Evo is the expansion so it will also work in that game
Did you copy both the UIDATA and Gamedata folder into the right map I.B.94? See what Paulo wrote: http://forum.racedepartment.com/169100-post7.html
mate, there's no standardised way of packing and distributing tracks... so you need to apply a l'il common sense when extracting them to the game... since each builder/converter does their own thing...
as mentioned, the root directory for race 07 (and all expansions) is: