Custom Taxonomy does not show up in WP Menu

Hi all,
I have created a custom post_type and a taxonomy to go with it. The taxonomy is not showing up in the nav-menus.php admin page?

Here is my code, perhaps I am missing something?

function products_register() {

	$labels = array(
		'name' => __( 'Products' ),
		'singular_name' => __( 'Product' ),
		'add_new' => __( 'Add New' ),
		'add_new_item' => __( 'Add New Product' ),
		'edit' => __( 'Edit' ),
		'edit_item' => __( 'Edit Product' ),
		'new_item' => __( 'New Product' ),
		'view' => __( 'View Product' ),
		'view_item' => __( 'View Product' ),
		'search_items' => __( 'Search Products' ),
		'not_found' => __( 'No Products found' ),
		'not_found_in_trash' => __( 'No Products found in Trash' ),
		'parent_item_colon' => ''
	);

	$args = array(
		'labels' => $labels,
		'public' => true,
		'publicly_queryable' => true,
		'show_ui' => true,
		'query_var' => true,
		//'menu_icon' => get_stylesheet_directory_uri() . '/product16.png',
		'rewrite' => array('slug'=>'products'),
		'capability_type' => 'post',
		'hierarchical' => false,
		'menu_position' => null,
		//'show_in_nav_menus' => true,
		'supports' => array('title','editor','thumbnail','comments')
	  ); 

	register_post_type( 'productpage' , $args );

	register_taxonomy("Product Categories",
					  array("productpage"),
					  array("hierarchical" => true,
							"label" => "Product Categories",
							"singular_label" => "Product Category",
							"rewrite" => true));

}