Downloads from here do not install like they do from the Steam workshop.
You'll need to extract the rar/Zip, place the rfcmp file into the Package folder that is found \steamapps\common\rFactor 2\Packages.
Open the launcher, search down the Locations list, high light it and install.
THAT WAS WHAT I was going to suggest next. That track is a package and has to be extracted to the packages folder and then installed from the rF2 launcher by clicking on the box icon. I just installed it and it was right there.