Portfolio items don't have the arrow to scroll

I want my portfolio items, when you click them and they enlarge, to have the right and left arrows so you can scroll through them, without having to keep going back to the thumbnails. I checked the “Check this box if you want add link to previous and next project” box for every item so far, but there are no arrows. Is there something else I need to do? Here is the link for the page.

http://www.mkillustrations.com/index.php/illustration/