To add a testimonial, go to your dashboard and navigate to Testimonials > Add New.
The title of the post will be the testimonial name, the featured image will be the headshot, and the body of the post will be the body of the testimonial.
To have your testimonials display in a widget area, go to Appearance > Widgets. Drag the “Showcase Testimonial Slider” to Front Page 3 widget area. Here are the settings I used for the demo:
For more information on setting up the front page widget areas, please see this page.