my problem is when i import my fbx into unity and add a mesh collider to it things still fall through. even after I applie rigid body and box collider to the object I am trying to keep on top of the mesh.
(it’s a landmass that I am trying to drive onto with a hovercraft.) but I keep driving through the mass. 
any input would be most helpfull. thanks in advance.
most of the time this is an issue of scale and speed. if your speed is so fast or your object is so small that the object passes through your terrain between time steps there’s no collision detected. in project settings → physics you can adjust the time step. checking too often can get cpu intensive though. i’ve found it’s best to model at 1:1 scale. you can also try bumping up your terrain’s tessellation.