Same problem. The NG Slideshow widget simply shows the spinner, no images.
For the JW imagerotator I've done the following:
Uploaded imagerotator.swf to both uploads/ and plugins/nextgen-gallery/.
On Gallery/Options/Slideshow tab:
1. "Search now" auto-located uploads/imagerotator.swf
2. Checked "enable flash slideshow"
3. Enabled navigation bar
4. Saved changes
With flash enabled, NG Slideshow widget now shows a black box with spinner. Again, images never display.
Something that may be related - I've installed the NGG Video Extend plugin. When I add its shortcode to an image description per the installation instructions, when I click on the image thumbnail, the lightbox pops up and says "loading..." The movie never plays.
Also tried the NGG monoslideshow plugin. Installed and activated. But when I added
[monoslideshow id=3 w=450 h=350]
(id 3 is a valid gallery ID) to a post, nothing displayed.
Looks like a flash configuration issue?