Twenty Twelve Integration Guide

Manual Integration: WP_nav_menu

Replace the following code in the header.php file:

<nav id="site-navigation" class="main-navigation" role="navigation">
   <button class="menu-toggle"><?php _e( 'Menu', 'twentytwelve' ); ?></button>
   <a class="assistive-text" href="#content" title="<?php esc_attr_e( 'Skip to content', 'twentytwelve' ); ?>"><?php _e( 'Skip to content', 'twentytwelve' ); ?></a>
   <?php wp_nav_menu( array( 'theme_location' => 'primary', 'menu_class' => 'nav-menu' ) ); ?>
</nav>

With the following –

<?php wp_nav_menu( array( 'theme_location' => 'primary', 'menu_class' => 'nav-menu' ) ); ?>