Adding the Main Slider to Homepage
1. Make sure you are using the "Drag & Drop Page Builder" template for the page:
2. When saving, there should be an option available for the Main Slider:
Select the categories you want to use, you can select multiple by holding on control/cmd button and clicking them (make sure not to select "Off" though).
Hit the save button.
3. Make sure that the categories contain posts that are marked as Slider Posts (that can be done while editing these posts):