Datasmith CAD importer does not "see" DWG files

In UE 4.26
Mac OSX Mojave 10.14.6
Datasmith CAD Importer & Datasmith Content plugins installed, UE restarted to use plugins.
Datasmith button is present in the top toolbar
“Import Unreal Datasmith File” hint shows when hovering the cursor over the DS button.
Click DS button and navigate to .dwg file exported from Vectorworks.
The .dwg file is greyed out, UE won’t open it.

Does the Datasmith CAD importer (beta) work in OSX Mojave 10.14.6?
Does another plugin need to be installed & running for Datasmith CAD importer to work?
Am I missing something?
Many thanks,
Martin