A little discovery.
I was trying to install a trial version of AC15 and kept getting the message
"The specified directory is an ArchiCAD directory. Please select a different directory, which is not an ArchiCAD directory"
. I quickly found
this ArchiCAD Wiki article but was still confused because I did not have have archicad directly in my applications folder.
... however
I did have three applications that I created with Fluid called 'ArchiCAD Help', 'ArchiCAD Talk' and 'ArchiCAD Wiki' - and apparently using the word 'ArchiCAD' when I named them is enough for the installer to think that one or all of them is actual ArchiCAD application
... so
When naming your new applications created with Fluid don't use the word 'ArchiCAD'. I have renamed mine 'AC Help', AC Talk' etc