When writing a page/post, you can use the follow tags:
For a slideshow : [slideshow id=x w=width h=height]
Example : http://nextgen-gallery.com/slideshow/
For a album : [album id=x template=extend] or [album id=x template=compact]
Example : http://nextgen-gallery.com/album/
For a gallery : [nggallery id=x]
Example : http://nextgen-gallery.com/gallery-page/
For a single picture : [singlepic id=x w=width h=height mode=web20|watermark float=left|right]
Example : http://nextgen-gallery.com/singlepic/
For a image browser : [imagebrowser id=x]
Example : http://nextgen-gallery.com/image-browser/
To show image sorted by tags : [nggtags gallery=mytag,wordpress,… ]
Example : http://nextgen-gallery.com/gallery-tags/
To show tag albums : [nggtags album=mytag,wordpress,… ]
Example : http://nextgen-gallery.com/albumtags/
To show random mages : [random max=x ]
To show the most recent added mages : [recent max=x ]
A further FAQ you can found here : http://alexrabe.de/wordpress-plugins/nextgen-gallery/faq/
And at least request your question here : http://wordpress.org/tags/nextgen-gallery?forum_id=10
