To upload a setup, users can log in and go to their garage. Then fill in the boxes and hit upload.
For the timebeing setup files will be manually approved. This is designed as a safety feature, to stop people uploading anything dodgy.
The uploader is in beta. I encourage you to email/pm/post here if you find a problem, or if theres a track missing from the list.
There is a detailed instruction manual here: http://www.netkarcentral.com/wiki/site-help/item/90-how-to-manage-car-setups
And you can view uploaded setups here: http://www.netkarcentral.com/files/car-setups
Hopefully we will get an uploader for tracks/skins/mods working in the coming days/weeks.