I'm using a child theme, and if I place the ngg-sidebar-widget containing the templates in the child theme directory, the plugin doesn't find them. It only works if I place ngg-sidebar-widget in the parent theme directory.
Could I suggest that you replace "get_template_directory()" in the plugin with "get_stylesheet_directory()"? That will find the child theme directory if one is being used.
Thanks for a useful plugin!