A very early version of Grasshopper is available in the Rhino for Mac WIP for testing and feedback.
Rhino 5 for Mac users have access to the Working-In Progress (WIP) builds. The most recent Rhino WIP contains a hidden command called ExplicitHistory.
To try out Grasshopper in the WIP build, just type "explicithistory".
Please post feedback and report bugs (there are many!) in this category.