Make sure you have:
-version S2 P2 or later,
-enabled InSim on your host with '/insim PORTNR' or started the host with the option '/insim=PORTNR',
-opened the InSim Port (UDP) in your router or firewall,
-closed any InSim applications you may have running.
1. Put it to /data/settings/ folder
2. Rename it to "car name_set name"... For example, if you downloaded set named "Formula_V8_blablabla", you have to rename it to "Formula V8_blablabla" same with other cars....
You have to remove the "_" from car name, if you didn't understand what i wanted to say ^_^