Hi Bob, the file probably contains either only all polygon mesh objects, or only all block instances - neither of which MoI reads from 3DM files.

If it's because they are block instances, then converting the instances into regular objects in Rhino and then resaving the file would then work, to do that open the 3DM file in Rhino, select everything and run the ExplodeBlock command, then save out a new version of the file.

If it's because the file contains polygon mesh objects only then it's a much more difficult problem because MoI is only designed to work with NURBS surface and solids objects and not polygon mesh objects. You would generally need to use a different kind of program, one that works on polygon mesh type data in order to work on models like that.

Hope this helps, - Michael