Nav Menu Display

Register your menus in functions.php like so:

if ( function_exists( 'register_nav_menus' ) ) {
    register_nav_menus(
        array(
            'my_main_menu' => 'Main Menu: description',
            'my_sub_menu' => 'Sub Menu: description',
            'my_footer_menu' => 'Footer Menu: description'
        )
    );
}

Add the menu to the template like so:

<?php wp_nav_menu( array( 
    'sort_column' => 'menu_order', 
    'theme_location' => 'my_main_menu', 
    'container_class' => 'main-menu' 
    ) 
); ?>

The function adds a container class, so you might want to consider trimming some of that markup.