This snippet is for the functions.php file

add_theme_support( 'menus' );

or

register_nav_menu( 'primary', __( 'Primary Menu', 'twentytwelve' ));
register_nav_menu( 'footer', __( 'Footer Menu', 'twentytwelve' ));

This is where you want the menu to appear:

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