This solved the problem for me. (I successfully downloaded and installed Affinity from the App Store on my iMac, but could not register.)
Please note that the "Application Support" folder mentioned above, contains a "space" in the folder name, so you have to put in between quotes in case you want to copy the afreg.cfg file. Once you copied the file to this folder and start Affinity again, registration starts by opening an internet link. Unfortunately that link could not open in Safari, but I was able to open it on Firefox. (Copy Paste) and then register. I successfully registered and got the 4 free filters.