Go to the Testimonials menu. Click on the Add item button at the top.
Set the title then you can create your content in the editor via the default WordPress editor or by switch to the Visual Composer Backend Editor. On the right you can set a category. Later on you can filter your slider to show only a certain category. Finally click on Publish.
You can simply add a testimonials section to your page by using the Testimonials Visual Composer element.