Vehicles, Terrains and Packs all work the exact same, it is only for organization. It doesn't matter.Depends what folder you are talking about and what type of mods you are talking about
Vehicles like trucks, boats, planes, cars etc go into the ''vehicles'' folder in the Rigs of Rods 0.4 folder.
Maps go into ''terrains''
Packs of vehicles sometimes don't work in vehicles, in that case dump it into ''packs'' (or extract the packs contents and dump it into vehicles, that sometimes works)
I am having the same problem there is no packs folder but there is a mode folder I put them in there and it is still not working
Documents\Rigs of Rods 0.4\modsis the new default place for mods. Just dropping the zip file in that folder should be enough. What mod are you trying to install?