The installation instructions in the readme.txt file indicate that the plugin should be copied to a folder called nggallery, but Alex must have removed that requirement since writing that as the code clearly allows the plugin to be installed in any directory below wp-content\plugins.  I initially installed it in the nextgen-gallery directory and it worked fine.

Before the plugin will work, you need to install JW Image Rotator in the NextGEN Gallery plugin directory.  You can get that from http://www.jeroenwijering.com/.  Once you’ve downloaded it, just copy the imagerotator.swf file to the NextGEN Gallery plugin directory.

Now you can activate the plugin.  You’ll notice that there are two NextGEN plugins – a NextGEN Gallery plugin and a NextGEN Gallery Widget plugin.  If you want to display images on your sidebar, activate the widget plugin as well.