Quote from LFSn00b :

PLZ tell me why i cant use those skins.

Because you don't have S2.

You can use them on your own car, but noone will see it online, and you can't see other people's skin online.
(People who have S2 can upload their skinfiles to a central server, which makes it possible to use those skins online)

Have you put them in the subdirectory data/skins ?
data/skins_x is for ones you want to see but don't want to use (i.e. not filling up your selection screen)
Then you need to setup a new colour selection (on left in garage) to set body colour, wheel colour etc, and attach the skin texture (click colours at bottom, texture selection below that).

theres a tutorial round here somewhere that takes you throu all these steps. Now where was it....?

http://www.lfs-gear.com/TUTORIALS/Tutorials.html

There...but then I'm sure you knew that really