Grevit 'Revit extrusion' not exporting

Hi

I'm trying to export a simple Revit extrusion using Grevit. When Grevit connects to Revit, I am choosing a conceptual mass family template (or is generic model better?). However, I then get an error message: 

Revit could not complete the external command. Contact the provider for assistance. Information they provided to Revit about their identity: MTT.

What would be causing this? The mass is a simple polygon extrusion. Is it something to do with admin privileges?

Thanks