I see a few displacement-related problems with the scene. First, there are two materials which are using the gravel bitmap:
[Color_A05], which is using MXM mode (in Scene Manager > Materials), but which has no MXM file linked, and
grindvloer, which also uses MXM mode, and which links to an MXM named
gravel.mxm.
The problem is that the face in your model that is supposed to be gravel has the
[Color_A05] material applied, rather than the
grindvloer material. Since
[Color_A05] has no MXM assigned, a generic Maxwell material is being generated for it, which uses the
gravel.jpg image specified in SketchUp material. So in your rendering, it will have the correct texture, but will not be a displacement material.
To correct this, I downloaded a random gravel displacement material from the MXM Gallery, which I think may be the same one you are using. It imported into SketchUp as a new material named
gravel, which is set to use MXM mode, and points to the newly-downloaded MXM file. The scene would be ready to render, except for one other problem: due to a bug in Maxwell 2.7, before I applied this material to your plane, I removed its materials, grouped it, and applied the gravel displacement material to the group. For more on how to deal with this bug, please read
here.
While the scene would now render with displacement, there is one more important improvement to make. In Maxwell, one big face is not going to produce the best displacement; it is better to subdivide the plane somewhat. For a discussion of what I mean, please see
here. You can see there the effect of subdividing the plane. To quickly do this in your model, I deleted the big face with the gravel material and created a new one, using
Draw > Sandbox > From Scratch, with a grid spacing of 0.05m.