Mesh Renderer and Rigid Body

I am a little confused and a little new to this :), I am using an object from protofactor the transport mech, I want to add a Mesh collider and a rigid body.

I put the object in the scene select it and add a rigid body, so far so good. but if I run this the object falls through the terrain. I then try to add a mesh collider to the object. the mesh is provided by profactor so I pick that and add it.

now the problem is the mesh is at a 90degree angle to the object. it looks like the exact shape but its not applied correctly.

so my first attempt was to put the mesh one level deep in the fbx hierarchy, this seem to work the mesh is now lined up and if I roll a ball into it bounces off all is good...

except when I turn on the walk animation I can see in the scene view that the mesh is stationary and does not move with the legs as they move.

how am I supposed to add both the rigid body and the mesh to this object?

thanks in advance for any help