Add category ID to menu items.

functions.php

<?php

/**
 * Category ID on Menu 
 * 
 * @author Bill Erickson
 * @link http://www.billerickson.net/code/category-id-on-menu-items
 *
 * @param array $classes
 * @param object $item
 * @return array $classes
 */
function be_category_id_on_menu( $classes, $item ) {
  if( $item->object !== 'category' )
		return $classes;
		
	$classes[] = 'menu-item-category-' . $item->object_id;
	return $classes;
}
add_filter( 'nav_menu_css_class', 'be_category_id_on_menu', 10, 2 );