So, once again, Reddit comes to the rescue (PreSonus, that's just sad!)....
I made a post on Reddit, got this response and it worked for me. See below:
Hello! I had the exact same problem yesterday and I’m happy so say I found the solution. For some reason, on a Mac, when you click ‘activate’ it doesn’t automatically generate the file path and folders to install the license key and files etc. Hence why nothing happens. You need to manually create the path as follows: click on your hard drive icon, then ... Users, Shared, Library, Application Support, then create a new folder called Presonus, open this folder and create another called Presonus Sphere and try activating again. This worked for me right away. Fingers crossed!