<?php
                    if(function_exists('wp_nav_menu')) {
                            wp_nav_menu( 'theme_location=menu_2&menu_id=nav&container=&fallback_cb=menu_2_default');
                        } else {
                            menu_2_default();
                        }
                        
                        function menu_2_default()
                        {
                            ?>
                            <ul id="nav">
                               <?php wp_list_categories('depth=3&hide_empty=0&orderby=name&show_count=0&use_desc_for_title=1&title_li='); ?>
                            </ul>
                            <?php
                        }
                    ?>