Hello Richard,
I am sure we can sort it out!
Based on your signature I assume you are on macOS 10.14. The first thing I would check in this case, is whether you have ever used Grasshopper on that computer before. When you first install Rhino, it is just a Drag-and-drop, but the system won't register that it is installed. You will need to open Rhino, start Grasshopper, and play around with it a bit, and then quit the application. When you quit the applications, several system level settings are created, that will tell the installer that you have Grasshopper and Rhino installed on your machine.
Please try this, then restart the computer and try installing the add-on again. If it didn't help, please get the
installer logs from this folder, and send them to me, so that I can take a look at what might go wrong. Also, please let me know what version of Rhino you have installed, and what version of ARCHICAD you are using.
Thanks,
Daniel Alexander Kovacs
Professional Services Consultant
GRAPHISOFT
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