Ahhh. Here is a bit of code that might help you:
<?php echo nggShowSlideshow($galleryID, $width, $height) ?>
Unfortunately, that only works with galleries. You would have to make a gallery for each post.
NextGen uses its own tables to store images. It’s completely separate from the wordpress Media Library.
If you attached images from the Media Library, you won’t be able to use any NextGen stuff with them.
You can also use a shortcut via the post editor, which can insert a gallery slideshow (as well as other gallery/album/image types) via shortcode. Again, you can’t do something that just automatically takes attached Nextgen Images and throws them in a slideshow.
There might be a plugin that can do what you want. The only other option is to code something custom.
NextGen Plugins
WordPress plugins