Image preview pop-up not working

I tried adding nextgen gallery plugin and the image doesn’t load in a pop window that I can’t scroll through, instead it loads in a page. When the page is clicked on it goes to an enlargement, there is no connection to the other images in the gallery and the only way to view a new image is to click the back arrow to the gallery. This is also true of a single image linked to itself. THis theme was sent up by a developer who is no longer available and I’m trying to finish the project.

Is there some basic jguery the controls this that isn’t a basic part of wordpress that he left out? I’m a little lost on this one.