You must put the tags in the template file: gallery.php (look for this file in folder /wp-content/plugins/nextgen-gallery/view/)
When you modified this file, you may want to save a copy of it to your theme directory (create a folder named /nggallery/ in your theme folder, and put the file in this folder, WP will look into your theme folder and find this file to use first) , and keep the original in the plugin folder, so that when you need to upgrade the plugin, your custom template won't be overwritten.
Here's an example:
I created a custom field called "Artist bio", so I put a line in gallery.php to display this field:
<p><?php echo nggcf_get_gallery_field($gallery->ID, "Artist Bio"); ?></p>
Hope that helps.