Hi Hamish - this happens because the MoI v3 beta now saves Rhino v4 3DM files, the previous versions of MoI saved Rhino v3 3DM files and previously the 3dmCurves2DXF utility only knew how to load Rhino v3 3DM files.

I've updated the utility so that it should be able to read any of Rhino v3, v4, or v5 3DM files ok now, so I think the updated utility should work properly for you now.

Can you please try the updated one, which you can download from the same spot as before which is here:
http://moi3d.com/download/3dmCurves2DXF.zip

Thanks,
- Michael