McNeel released Rhino 6 for Mac recently. Unfortunately, our KeyShot plugin does not work with Rhino 6 on Mac at the moement (8/28/19). There is no ETA yet. Please stay tuned to our plugins website (https://www.keyshot.com/resources/downloads/plugins/) and to our newsletter (https://blog.keyshot.com/topic/news) for updates: https://www.keyshot.com/resources/downloads/plugins/
For now, you may import your .3dm files directly via File > Import inside KeyShot. If you are running an older KeyShot version, you may export your models as "Rhino 5" or as .STP files. For help troubleshooting import errors, please click the link below or email us at support@luxion.com.
Import 3D Models Troubleshooting Guide (https://help.keyshot.com/customer/en/portal/articles/2568763-importing-3d-models)
Luxion Support
The plugin for Rhino 6 to KeyShot 8 has been released and can be found on our plugins website: https://www.keyshot.com/resources/downloads/plugins/