Your gallery is sourced with images from the Media Library and WP-SimpleViewer will display all images attached to the post containing the gallery. It looks like there are no images attached to the post containing your gallery.
Try the following to check how many images are listed as being attached to the post containing the gallery.
Edit the post containing the gallery.
Click the 'Add Media' button.
Select 'Insert Media' from the menu on the left (selected by default).
Select the 'Media Library' tab (also selected by default).
Choose 'Uploaded to this post' from the drop down menu.
See how many images are listed.
You could also go directly to your Media Library and check which images are listed as being attached to the post containing the gallery (in the 'Uploaded to' column).
with the images below it, not in it
This sounds like you may be clicking the 'Insert into post' button. This is not necessary (and will insert the images directly into the post). The images need only be attached to the post.
The procedure is essentially:
Create a SimpleViewer gallery in a page or post.
Click the 'Add Media' button above the toolbar.
Drag and drop images into the media window.
Wait for the uploads to finish and close the media window with the cross at the top-right.
Publish the page or post.
SimpleViewer Support Team