Please clarify: how to use NextGen thumbnails

I’ve looked at the resolved topic on this subject, as well as the Galleryview page on custom thumbnails, but I am not clear on what needs to be done. I see where to add the code (I think) in gallery-galleryview.php, but I assume that somewhere “thumbnailURL” needs to be defined? Sorry, I’m not a php or javascript expert so I’m a bit dense at this level of detail.

http://wordpress.org/plugins/nextgen-galleryview2/