So Design Spark wont save out 3D models in that sense. (You can save polygonal data like OBJ or Sketchup etc. Not sure if CAMBam etc reads that data) Or an STL "Dumb solid".
The dxf export is going to be a 2d option that basically "Flattens" things out. While you are in "3D Mode", it will flatten from "Perspective". Your file opens like this in my apps and I don't see anything "Missing" so to speak, other than I don't think that was what you are after.
Here are 2 files back from Design Spark. An STL and a DXF that was exported fro "PLAN View" Go into "Sketch Mode" then down on the bottom enter "Plan View" to do your export to 2d dxf.
If your CAM programs aren't opening this DXF as good 2d geometry, you may try running it through some other app then back out...
Anyway...