I use this theme on irontroll.com.
If you go to the Customizer section for your blog, the option to activate the slider is found under THEME, which is the last item on the Customizer menu.
Once you click on THEME, scroll down and you’ll see the slider activation and the options for it.
Once activated, go back to your Customizer menu and select the FEATURED CONTENT to select what tag you want to feature in your slider. Once you add that tag to blog posts they’ll show up in the slider.