I think your trying to create a csm mod using the Extra Stuff menu in CSM.
That only works when you have a complete carset working.
But lets say your not using that ... then you go to
name-of-the-mod\Data\Cars and you will see a file called "config",
inside you will see the following:
[Main]
Format=
[Config]
SkipBuild =
Dir1 =
Dir2 =
Dir3 =
Dir4 =
Dir5 =
Dir6 =
Dir7 =
Dir8 =
Dir9 =
Dir10 =
Dir11 =
In each "Dir" put the file location of each car and save the file. DO NOT PLACE ALL CARS IN ONE FOLDER !!!!.
For each car create a folder.
Try that and let me know.