There's two ways of doing this. The first is to use a call to the function that displays the viewer and the second is to enable the use of shortcodes in widgets.

add_filter('widget_text', 'do_shortcode');

The above code has to be put into your theme's functions.php file. Then you can use shortcodes in widgets.

echo nggShowSimpleViewer( galleryID, Width, Height, Link )

Now this bit let's you output SimpleViewer directly into a template. You have to wrap it inside php tags and adjust galleryID and the rest of the parameters to your needs. Here's an example without a link:

echo nggShowSimpleViewer( 1, 250, 175 )

The new version will do that. Try the beta version that's out although it still has a few bugs…