OK, so I finally got back to the studio and was able to look at the MXM that you are using --the problem is one of precision.
What you need to do is set the displacements "subdivision" setting higher -- or simply use the adaptive setting (my preference).
But before you do that, make sure you understand something very basic for successful displacement in Maxwell -- the more subdivided the geometry you are applying the displacement material to is, the faster and better the material will render.
I prefer to use the SketchUp Sandbox tools to quickly generate highly subdivided planes for displacement materials... in this example I made a 1m x 1m plane with subdivision every .01m (I did say highly subdivided
)... then I smoothed the co-planar edges and applied the SketchUp material I made.
I set the displacement for adaptive at 1cm with the offset at 0.5 --therefor in SketchUp I also dropped the ground plane to -0.005m.
This combination of settings allows for very fine detail that renders fairly quickly (this render took about 9 minutes) at the cost of a geometrically heavy SketchUp file.