In the URL part insert the ID of the Stack LightBox you want to show.
Add a Custom Atribute and name it 'class' with value 'mb_lightbox'.
If you want to group it in order to cycle through a gallery of lightboxed elements you have to add another attribute: 'rel'
All elements that should appear grouped need to have the same 'rel'-value, for example 'gallery1',...
If you don't add the rel attribute all non 'rel'-led elements will be grouped too, so if you want one lightbox to appear non-grouped you need to give it a unique 'rel'-value.
Use the attribute "data-order" and an integer to create an ordering for your grouped lightboxes.
You can display another webpage lightboxed if you setup an URL and set the class to "mb_lightbox iframe"
You can display YouTube videos lightboxed if you setup a link to the video and set the class to "mb_lightbox_youtube"