The above answer by @wim is correct.
Grasshopper for Mac is only available in the RhinoWIP for Rhino 5 for Mac. Since Rhino 5 for Mac - the full Commercial release - and the RhinoWIP share the same license key, it is possible to “evaluate” the RhinoWIP (and thereby test out Grasshopper for Mac in it’s current Work-In-Progress state). Keep in mind, you should not use the RhinoWIP for production work and Grasshopper for Mac should not be considered “done” by any stretch of the imagination.