Having recently acquired an iMac and tried to install Sigma Photo Pro I now understand the angst - it works reasonably well on PC by comparison. I don't know why it won't go into the same place as other applications but the most annoying thing (so far) is that the browsing pane on the left hand side only shows part of the list of folders and there is nothing to toggle/navigate up and down with - any clues.
Perhaps I should just use SPP on my windows laptop to import 3xf files and convert to TIFF.