To install skins copy them to C:\Users\<Your Username>\Saved Games\DCS\Liveries\<Plane Name>
When in your plane (on the tarmac) press Left-Alt + ' to enter the load-out menu and select the livery from the drop down. If a livery that you now you have installed is not showing up, the livery may be assigned to a specific set of countries and you may not be a country of that plane in the game.