Hi,
in your dashboard you need to click on “Testimonials” and Add New. Write Client name, url, company and add Testimonial category(you can do that from Testimonials/Category or directly when you add new testimonial you can add category in right box).
Then go and edit you page, add element named “Testimonial slider”. In Testimonial slider element you have option to put Testimonial category or just leave empty field. If you add category there will be only testimonial with that category, but if you leave empty field all testimonials that you created will shows.
Let us know if you’ll need anything else :)
Kind regards