<?php wp_nav_menu( array('theme_location' => 'primary or whatever you set it', 'sort_column' => 'menu_order', 'menu_id' => 'nav', 'menu_class' => 'clearfloat', 'container_id' => 'header_nav' ) ); ?>
Codex if your friend! http://codex.wordpress.org/Function_Reference/wp_nav_menu