Put the .zip file in My Documents/Rigs of Rods/packs . Then open RoR and select the track. If you don't have trucks, follow this link ( http://sim-monsters.com/index.php?/files/file/177-sim-monsters-truck-pack-v3/ ) and put that file in the packs folder as well. After you have opened the track, there should be a menu that lets you select a truck.