First, please bear in mind that we provide the FatFreeCart.com service on a strictly do-it-yourself basis with no support whatsoever (that's a major reason it's free :^), so you're expected to figure it out and troubleshoot any issues you may have with that on your own. If you decide to upgrade to an E-junkie cart subscription (aka "FatFreeCart Pro"), that does of course include support. Since we don't normally provide support for FatFreeCart, our support team isn't very familiar with how it works, but we can try to answer your question this time.
I was able to get it to work, albeit a bit clumsily, by adding the script portion of View Cart code in the page being shown in the lightbox overlay. However, that actually opened a new lightbox overlay inside the original overlay's iframe, rather than just re-rendering the original overlay, so this solution is obviously not ideal.