You can add testimonials by going to Testimonials -> Add New.

In a first step, you'll have to add the client name in the title field, and the testimonial itself in the post editor entry field:

Next, you need to scroll down to the "Testimonials Options" panel, where you can add the testimonial to the rotator you created earlier, you can select a star rating, you can add a job title or company, and of course, you can add client photo:

Once you are done, just click the "Publish" button to save the given testimonial.

Now you'll have to repeat the process to add more client testimonials.

Finally, after you've added a few testimonials, you are ready to add them to your site.

That's the next step ...