nextGen Gallery Slideshow

I will presume you have uploaded and activated the plugin.
Add a new Gallery via the Gallery dropdown in the Navigation of WP.
then add images to that gallery (Make note of the ID) and once that is done you can either manually add it into the page as per the readme, this is where the ID will be needed.

Alternatively create a new page, click the NextGen Button on the Kitchen Sink icon list at the top of the page. Select the Gallery and Slideshow option, Insert. It is here you can also specify the Width and Height.