Scroll buttons not showing (how to config)

I am using Custom Scrollbar for a site of a client. This site runs on a large touchscreen but due to hardware limitations the drag-options are limited. Therefore I would like to use the scroll buttons, but how do I change the code so these buttons appear.
Live site: http://infocenter-nijenbeek-v2.va-concept.nl/historie/

I have now added two separate buttons which could trigger a javascript for scrolling but prefer the scrollbar-buttons instead.
Hope to hear from you.

Kind regards, LB