We just chatted over Google Meet, and @esjones showed me what was going on.
It turns out that his screen was perfectly sized for the “Where would you like your PhotoStructure library stored?” section to just fit on the screen, and the placement of the “Select” button next to the custom library directory was positioned where the “next” button would be.
I’ve reworked the help text on the custom directory field, and added “Back”/“Next” buttons to the bottom of each card to hopefully avoid this confusion in the future:
Thanks for your time, @esjones !