Hi Tom, an STL file only contains triangle mesh data, not CAD surfaces and solid data.

Probably the STL converter will generate a polygon mesh object in the Rhino file, and MOI will not read those types of objects.

There is a converter for 3DM Rhino v6 to Rhino v5 converter available here (both Windows and Mac):
https://moi3d.com/resources#Rhino6/7/8_3DM_file_format_to_Rhino5_3DM_file_format_(MOI3D_compatible)_converter

But I think that is only one part of the problem, you will still have the problem that an STL file only contains polygons in it and not structured CAD data like spheres, cylinders, and smooth surfaces.

There is some more info on polygon mesh data versus CAD model data here:
https://moi3d.com/faq#Q:_Why_can_I_export_to_.obj_format.2C_but_cannot_read_.obj_format_into_MoI.3F

Some recent discussion on the forum here:
http://moi3d.com/forum/index.php?webtag=MOI&msg=12004.1

- Michael