Where I can download the Rhino SDK for Mac?
Sorry, no such thing yet. The only customization currently available on Mac Rhino is Python.
Is an OSX C++ SDK something that is feasible, given the way Rhino osx is using MFC?
Probably, but we don’t really know yet. We are going to put an initial effort into making a RhinoCommon based SDK available since we already know how to do that and there are only a few pieces missing before this can be considered done for 5.1.
A C++ SDK as you already know can be a bit more “finicky”. We are going to start by dog fooding an SDK internally by making PanelingTools a plug-in. Once that is done, we’ll have a better idea of what is really involved for properly supporting a C++ SDK.