Hi Paul,
Looking at your image it doesn't have the necessary "Add Mono" or "Add Stereo" buttons on the UI, which is not normal for the Studio One 3 Professional demo, so there is something unusual going on there. Perhaps you are running Studio One 3 Prime. Even if it is Prime you should still have access to the first two I/O on it.
If your OS is Sierra you try using an aggregate device as described in the Apple support article below to see if it allows the I/O to show up after that...
https://support.apple.com/en-us/HT202000