Additional navigation won't register in site

I’ve added an additional navigation to the twenty ten theme in the footer. I’m adding the code:

<?php wp_nav_menu( array( 'sort_column' => 'menu_order', 'container_class' => 'footermenu' ) ); ?>

into the footer.php file, and adding the menu named “footermenu” into the admin panel. It won’t seem to take the menu, only the main menu. What am I missing here?