Just a note if after installing the next-gen gallery monoslideshow, and after purchasing the monoslideshow.swf, your preview gives you an error 1090 - check the file names of the images in your gallery.
I had a photo named with an apostrophe and a file named with an ampersand. I deleted each of those photos from the next-gen-gallery, renamed without those characters and re-added them to the gallery. Then the monoslideshow preview worked fine after this step.
I'm sure there are other characters not allowed, but those were the two that I found.
To figure out which image file was causing the problem, The error that showed up over the preview was a URL. I directed my browser to that URL and it told me which image it was choking on so that helped me to figure out which character was the culprit.
I hope this helps save some frustration for other new users