This code will help you get the URLs for a plugin inside WordPress Must Use directory. This makes it easy for you to use them throughout your plugin in order to reference things like CSS and JavaScript files.

wordpress-mu-plugin-urls.php

<?php
/**
* Define URLs for a Must Use Plugin
* Remember to change EXAMPLE to your plugin prefix
*
* @author FAT Media <http://youneedfat.com>
* @copyright Copyright (c) 2013, FAT Media
* @license GPL-2.0+
*/

add_action('init', 'example_plugin_urls');
/**
 * Hook your URLs to init to  to ensure that the "plugins_url"
 * filters are already hooked at the time the function is called.
 */
function example_plugin_urls() {
	// Define the Main Plugin URL (Replace 'example-plugin' with your plugin folder)
	define( 'EXAMPLE_PLUGIN_URL', plugins_url( 'example-plugin' , dirname( __FILE__ ) ) );
	// Define the Lib URL (Could also be includes, classes etc.)
	define( 'EXAMPLE_LIB_URL', EXAMPLE_PLUGIN_URL . '/lib' );
	// Define URLs Inside the Lib Directory
	define( 'EXAMPLE_CSS_URL', EXAMPLE_LIB_URL . '/css' );
	define( 'EXAMPLE_IMG_URL', EXAMPLE_LIB_URL . '/css/images' );
	define( 'EXAMPLE_JS_URL', EXAMPLE_LIB_URL . '/js' );
}

A full tutorial on defining and using plugin URLs in a WordPress Must Use plugin can be found at http://youneedfat.com/get-the-mu-plugin-directory-url/