Twenty Eleven Integration Guide

Manual Integration: WP_nav_menu

Replace the following code in the header.php file:

<nav id="access" role="navigation">
   <h3 class="assistive-text"><?php _e( 'Main menu', 'twentyeleven' ); ?></h3>
   <?php /* Allow screen readers / text browsers to skip the navigation menu and get right to the good stuff. */ ?>
   <div class="skip-link"><a class="assistive-text" href="#content"><?php _e( 'Skip to primary content', 'twentyeleven' ); ?></a></div>
   <?php if ( ! is_singular() ) : ?>
   <div class="skip-link"><a class="assistive-text" href="#secondary"><?php _e( 'Skip to secondary content', 'twentyeleven' ); ?></a></div>
   <?php endif; ?>
   <?php /* Our navigation menu. If one isn't filled out, wp__nav_menu falls back to wp_page_menu. The menu assigned to the primary location is the one used. If one isn't assigned, the menu with the lowest ID is used. */ ?>
   <?php wp__nav_menu( array( 'theme_location' => 'primary' ) ); ?>
</nav>

With the following –

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