You put manually downloaded settings in your LFS > data > settings folder. They will then appear on the right when you're in the garage.
To send them to others in game, you press N to bring up the connections list and then click on the SS next to the person's name you want to send that set up to. It will say Setup Sent.
To receive a setup that someone has sent you, you again press N to bring up the connections list and you'll see that one of the S's in SS is now highlighted. You click on that and you'll get a box come up for you to name and save the setup.