I ran in to the same issue. Tried to paste the file path mentioned by Dan, however the text turns red and I cannot confirm it:
Also tried the workaround and follow the file path manually however GrasshopperPlugin.rhp is greyed out…
Is there something that I have overlooked?
Hey,
The absolute unix path pointing to the KangarooSolver.dll for the latest version of Rhino 7 is:
/Applications/Rhino\ 7.app/Contents/Frameworks/RhCore.framework/Versions/A/Resources/ManagedPlugIns/GrasshopperPlugin.rhp/Components/KangarooSolver.dll
However, since we’re dealing with some sort of Windows workflow here that haphazardly also seems to work on macOS, you need to get rid of the backslash that escapes the whitespace between .../Rhino\ 7.app/.... Naked whitespace is not allowed on unix-based systems, is considered a special character, and as such needs to be escaped with a backslash.
My guess is that this confuses the C# “path parser”, since it expects only back- or forward slashes. This dialogue doesn’t seem to be fully ported for good multi-platform support. It still needs some love!
I’ve tested it and am confident that the following path should work (if you’ve installed Rhino in the default location)!
/Applications/Rhino 7.app/Contents/Frameworks/RhCore.framework/Versions/A/Resources/ManagedPlugIns/GrasshopperPlugin.rhp/Components/KangarooSolver.dll
Previously, Rhino was called “Rhinoceros.app” by default, which didn’t pose an issue.
Thank you for detailed answer! 
this path worked!
Have a great weekend!
Rudi
Kein Problem! Dir auch ein schönes Wochenende! 
Yess! 
Worked on Macbook m1 pro Monterey Rhino7
The path to KangarooSolver.dll on Mac Os X for WIP V8
/Applications/RhinoWIP.app/Contents/Frameworks/RhCore.framework/Versions/A/Resources/ManagedPlugIns/GrasshopperPlugin.rhp/Components/KangarooSolver.dll
