Hello Aur,
Thank you so much for using our theme Corporate Plus Pro. Yes, testimonial is slider in our demo. If you want the same slider on your testimonial too, you have to select the parent page having two or more child pages on that testimonial widget. This theme is Parent/Child theme based theme. So, select the parent page and its child pages will display as a testimonial slider. Please follow this blog post for how to create child page. http://www.acmethemes.com/blog/2016/04/how-to-create-child-pages-sub-pages/
If you want to make the site as demo, please have a look here. http://www.acmethemes.com/blog/2016/04/corporate-plus/
If you face any problem, let us know, we will do our best to solve your problem.
Regards,
Acme Supports