Thank you, RV. I'm starting from scratch with Blender, and all I have been able to do so far is import the OBJ. As for having the mesh in face mode, selecting faces, creating a new group, assigning polys to it, etc.... well, I'm usually pretty good at finding my way around new applications, but I'm searching the Blender UI now and I'm not seeing how to do any of those things! So first of all I have to Read The Fine Manual (if there is one).
A job for tomorrow, I think... or rather, later today.

"If I were a shadow, I know I wouldn't like to be half of what I should be."
Mr Otsuka, the old black tomcat in Kafka on the Shore (Haruki Murakami)