The fact that I’m using Dash 2 may be why the RhinoCommon link can’t be added in the way described– it yields an alert about not being a valid feed. Instead one can follow the link contained in the XML file, uncompress the .tgz, and double-click the resulting docset to install it in Dash. That gets you RhinoCommon 1.5.0.
Since then we’ve gotten a commercial release of Rhino for Mac, RhinoCommon running on Mac, Eto UI forms, etc. and so it would be great if there was a way to get the same set as what’s online.