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.
Thank God for that, otherwise Slifko would get knocked out by Bad Habit. Have you seen how many people are on there just to tell them to send Joe to Vegas?
EDIT: SlifKo not SlifCo