Hi mk, there are 2 different kinds of STL files, there is a text file version and a binary version. I think the STL import plugin only supports the text version type so make sure you have that kind of STL and not a binary one.

Note that in general it's not very workable to import STL geometry into MoI because STL files only contain faceted triangle mesh data while MoI is designed to work with CAD surfaces and solids and isn't a polygon modeler.

- Michael