After you purchase and load the SOS plugin head to the left side panel and click on SOS.
You can import the 5 sample layouts SOS provide in the json file that comes with the plugin or you can build your own layout from scratch.
When beginning your design for the modal you need to save your new layout as Layout > SOS Layouts
If you want to use one of the sample layouts you then select Choose a Premade Layout, the SOS layouts will be available.
IMPORTANT – If the loading symbol hangs for ages head to Permalinks and save.