Thanks for your trouble Ian. I've had so much trouble configuring GrandOrgue that I've decided to go with Hauptwerk just for their auto detect configuring. Its a shame as I have everything working except pedalboard, expression pedal and couplers. I use a 2014 Mac mini with 16GB Ram, 1TB SSD drive with Catalina. For some reason GrandOrgue have left out the Couplers on the midi objects in my version, ie. instead of I/II or I/P it just has Coupler 1, 2, 3, 4, 5, but how do you configure them? Cheers and thanks for trying.Firstly, a warm welcome to our Forums.
I think a tiny bit more info about your Mac would be much appreciated. Model, year, Operating System (OS) etc.
As regards GrandOrgue setup, the links below may help. The second link has embedded in it a further link to MacOS.
Hope these might be a starting point for you?