The Model is invisible in Editor/Model Viewer or ingame?
Is the prop visible without textures?
In the first case, it might be related to the face normals.
Make sure that the polygons face normals are pointing in the right direction.
In C4D select all polygons in Polygon mode, orange tint means frontside, blue tint is the backface.
Right click on the selection to bring up the context-menue where you have "align normals" and "reverse normals" commands.
Make sure all faces on the frontside are tinted orange.
Second case would point towards a missmatching prop type (e.g. a prop compiled as prop_physics used with a prop_static entity).
Third case would be an issue with the material settings, e.g. translucence or alpha enabled with a black alphachannel or wrong shader.
(please post your vmt in that case)
On a sidenote, a mountain prop that is used in the skybox won't need a collision model at all, you'll never physically interact with it anyway.
Just to rule out a few more common mistakes:
- make sure that the pivot of the mesh is reset to the world origin (in Axis modify mode (press L), set position to x=0 y=0 z=0)
- make sure the scale of the object is right.
- if you scaled the object in the coordinate manager (any other value then 1 in size xyz), make sure to freeze the transformations.