I finally got the file, it's too big and was stucked in one of the e-mail servers.
Regarding the surface, normals are inverted but for some reason they cannot be flipped: neither using the Normal modifier or Editable Mesh tools. I found this trick in other Max forum:
1) Create a box (or any geometry) in the XY plane.
2) Collapse it to an Editable Mesh.
3) Turn on Selection->Show Normals to check normals visually.
4) Attach the surface to the box mesh.
5) Select all faces and click Surface Properties->Normals->Flip
6) Remove the box element.
Optionally i also suggest to apply Vertices->Weld (faces are isolated). Next, you can assign the surface in Forest as usual.