Adding a testimonial is best done once you have logged into your account. A guide to logging in can be found here.
Once you are logged in click on login and support and select View testimonials from the menu. You will be shown a page with the testimonial catagories. Click the button "add a testimonial", at the top of the list.
In the product drop down list select the product your testimonial relates to, and type your comments in the comments box.
When you are done click in the box to confirm you are a human and then click on the post testimonial button.
Thats all there is to it.