Dave, you need to delete https:// from the field where you enter the link (below the testimonial text).
After you press Update, it will appear again – doesn’t matter, it will work on the page. Also if you will have some troubles with symbols around the link, simply open and press Save testimonialslider/css/jquery.bxslider.css (Plugins -> Testimonials Slider -> Edit -> right panel with a list of files, find and open it)