After the installations, insert the followings code in your post or page to display your picture accordingly.

For a slideshow :

Example : http://nextgen.boelinger.com/slideshow/

For a album : [album=id,extend] or [album=id,compact]

Example : http://nextgen.boelinger.com/album/

For a gallery :

Example : http://nextgen.boelinger.com/gallery-page/

For a single picture : [singlepic=id,width,height,mode,float]

Example : http://nextgen.boelinger.com/singlepic/

For a image browser : [imagebrowser=id]

Example : http://nextgen.boelinger.com/2007/06/16/new-tag-picture-browser/

Read as startup : http://hqcode.com/?page_id=54

A further FAQ you can found here :http://alexrabe.boelinger.com/wordpress-plugins/nextgen-gallery/faq/

To integrate a widget into your theme manually, use the follow code in your theme:

For slide show: 

<?php
if (function_exists(’nggSlideshowWidget’))
{ nggSlideshowWidget($galleryID,$Width,$Height); }
?>

For random images :

<?php
if (function_exists(nggDisplayRandomImages))
{ nggDisplayRandomImages($number_of_pic,$Width,$Height); }
?>

For recent images :

<?php
if (function_exists(’nggDisplayRecentImages’))
{ nggDisplayRecentImages($number_of_pic,$Width,$Height); }
?>

Source: http://wordpress.org/extend/plugins/nextgen-gallery/