You can find that information in the readme of the downloaded cars&tracks.
If not, then:
If you downloaded a track and you only have one folder in it (with the track's name f.e.), you need to copy it to your Race07/Gamedata/Locations folder. If you have the online version you'll probably have to create those folders.
Cars (and most of the tracks), already have the right structure (so you have Gamedata, and sometimes UIdata too after you unzipped it), so you only need to copy them to your Race07 folder and they should work fine.
At least this is how it works for the online version, but I think it must be similar for the offline version too.