add_filter( 'wp_nav_menu_items', 'add_nav_menu_items', 10, 2 );
function add_nav_menu_items( $items, $args ) {
ob_start();
?>
<ul>
<li><a href="#">PRODUCTS</a>
<ul>
<li><a href="<?php echo esc_url(get_category_link(get_cat_ID('Sci-Fi')));?>">SCI-FI</a>
<ul>
<?php query_posts( array ('post_type'=>'scifi','showposts'=>-1,'order'=>'ASC') ); while ( have_posts() ) : the_post(); ?>
<li><a href="<?php the_permalink(); ?>"><?php the_title(); ?></a></li>
<?php endwhile; wp_reset_query(); ?>
</ul>
</li>
<li><a href="<?php echo esc_url(get_category_link(get_cat_ID('Drama')));?>">Drama</a>
<ul>
<?php query_posts( array ('post_type'=>'drama','showposts'=>-1,'order'=>'ASC') ); while ( have_posts() ) : the_post(); ?>
<li><a href="<?php the_permalink(); ?>"><?php the_title(); ?></a></li>
<?php endwhile; wp_reset_query(); ?>
</ul>
</li>
<li><a href="<?php echo esc_url(get_category_link(get_cat_ID('Horror')));?>">HORROR</a>
<ul>
<?php query_posts( array ('post_type'=>'horror','showposts'=>-1,'order'=> 'ASC') ); while ( have_posts() ) : the_post(); ?>
<li><a href="<?php the_permalink(); ?>"><?php the_title(); ?></a></li>
<?php endwhile; wp_reset_query(); ?>
</ul>
</li>
</ul>
</li>
</ul>
<?php
$products = ob_get_clean();
return $items . $products;
}