Do you have the plugin installed already? If not there are some instructions here:http://www.concrete5.org/marketplace/how_to_install_add_ons_and_the...

If you already have it installed you can create testimonials in the backend and add a "Testimonials Block" to your page. If you are new to concrete5 I would suggest taking a look at the documentation section of this site, they have some great resources to help you better understand how concrete5 works.