To use the 'Lookbooks Slider element' you first need to create a lookbook - see how to do so here.
You must ensure that you have set a featured image for your Lookbook as this will be the image used in your slider.
Once you have created a lookbook, you can now add a ‘Lookbook Slider element’ to your desired page using the WPBakery Page Builder.
In the Lookbooks slider popup, type in the lookbook names you want to display and select them. You can add as many lookbooks as you would like to the slider. Once complete 'Save changes'.
See it on the demo site.