@ 51thPolux,
You need to place your "Skin" folder with the games Liveries folders here :
\Eagle Dynamics\DCS World\CoreMods\aircraft\FA-18C\Liveries\FA-18C_hornet
Then go into the "View" menu in model viewer, select "Dialogs" , then select "Liveries " Dialog, then if you have placed your "Livery" correctly you will see your livery listed, then select it to see it mounted on the model.