Oh, something good has happened. I read in SketchUcation that I should rename the extension of the zip file from ZIP to RBZ.
Did this, then tried to install it via the extensions manager…it worked!
Then it said, for both files, that they were “unsigned”
Changed the settings to “Approve unidentified extensions”.
This seems to have worked, ConicCurve InFace is listed under the Extensions tab.
Have replied to my own email because I don’t wish to waste anyone’s time, and maybe this helps another newbie.
Now I “only” 
have to learn how to use the program and the plugin.
Many thanks, Carmen