So, go in your series folder where you put the cars with Winrar and look for files that end in .car, these are the actual car files. These files should be in the "cars folder" within the "mod folder" (for example, MENCS19 is a mod folder, within that folder is the cars folder). If the car files are just in the series folder or just the mod folder, they are in the wrong spot.
If they are .car files in the cars folder, then check to see what their physics are. Physics are the extension just before .car: they can be .cup, .gns, .cts, or .pta (there are others but it is unlikely that your carset is anything other than one of those 4). If the carset cars are different than the cars that were already in the folder, then you have the wrong physics.
For example, if a car in the carset you are trying to install is lets say 3_earnhardt.cts.car and a car that came with the mod is lets say 4_harvick.cup.car, then you are trying to put a car with cts physics into a mod that uses cup physics.
Check those two things and get back with us.