Change Slimbox2 Caption Source

I am using NextGen Gallery Custom Fields to manage caption data for my photo galleries. Can someone please explain to me exactly where to place/change the code so that when the slimbox is opened, instead of the caption being pulled from the “description” field, it is pulled from the custom fields instead?

The page is http://orenstransitpage.com/wordpress/grand-central/united-states/new-york/ithaca/tcat-orion-is/ (very much a work in progress however).

http://wordpress.org/extend/plugins/wp-slimbox2/