Hi Janne & Carlos, I've got a Windows version of a standalone converter here:
https://moi3d.com/download/Convert3DMToRhinoV5_win.zip

When you run it, you can pick a Rhino v6 or v7 .3dm file, and it will generate a v5 .3dm file. It will append a _v5 onto the filename, so SomeName.3dm given as input will generate SomeName_v5.3dm for the output and the _v5 one can then be loaded into MoI.

I'll also make a Mac version in a bit here.

- Michael