Oh, it's totally not intuitive but you can access the settings to edit/import UCI engines when you click on the Profile icon on the top left.
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Dear chess enthusiasts,
I have managed to import HIARCS as a UCI engine into the (beta) Chessnut desktop app for Mac.
It's not super stable (the Chessnut app) but it works.
The command line HIARCS engine is installed into the /Applications folder: