I would propose the following steps for you if you want to test that:
- Open Internet Explorer
- Go to Racedepartment.com, to download some skins
- Open your windows explorer
- Go to the DATA folder of your install, and browse into:
\Userdate\{profilename}\Settings\
- Open the rfactor2 game
- Select All Cars and Tracks
- Select the car class you downloaded the skins for
- click on Tuning (Middle bottom of the screen)
- Click on the Create folder button
- Switch to your explorer. Note that the game made a subfolder here with the class name
- Enter that class folder
- Note that there is a folder now with the corresponding car number you had open in the game
- Place the skin here. Make sure it has the ALT word in the filename, and that you do not have a .zip file by accident
- Go back to the game
- Click the reload button
- Now click on the arrow next to the skinname in the right bottom.
- The skin you just placed in the created folder should be selected now.
I might have missed a step, as I made this list from my head and have not checked it in the game. If you still have issues, let me know. I will send you a working and tested skin so you can eliminate that as a possible cause.