Forgive me for not checking the github first. I presume this is a known issue? I copied in all of my existing slots, and they seemed to save correctly. But when I return to the Slots GUI window, they don’t re-appear. 
Well, look at that - there’s an issue. 
Did you check the dropdown menu? I missed this the first time aswell
Yup, sure did. Nothing there to select.
Solution is in the Github issue #34 There should be a ‘slots’ sub-directory beneath your profile directory and then it creates and saves slot files correctly.