If i understand your question correctly, the first thing you would need to do is, go to your menus /wp-admin/nav-menus.php (screen options) and checkbox (tick) on the description.
and this is how your walker class should look like
class Menu_With_Description extends Walker_Nav_Menu {
function start_el(&$output, $item, $depth, $args) {
global $wp_query;
$indent = ( $depth ) ? str_repeat( "\t", $depth ) : '';
$class_names = $value="";
$classes = empty( $item->classes ) ? array() : (array) $item->classes;
$class_names = join( ' ', apply_filters( 'nav_menu_css_class', array_filter( $classes ), $item ) );
$class_names=" class="" . esc_attr( $class_names ) . '"';
$output .= $indent . '<li id="menu-item-'. $item->ID . '"' . $value . $class_names .'>';
$attributes = ! empty( $item->attr_title ) ? ' title="' . esc_attr( $item->attr_title ) .'"' : '';
$attributes .= ! empty( $item->target ) ? ' target="' . esc_attr( $item->target ) .'"' : '';
$attributes .= ! empty( $item->xfn ) ? ' rel="' . esc_attr( $item->xfn ) .'"' : '';
$attributes .= ! empty( $item->url ) ? ' href="' . esc_attr( $item->url ) .'"' : '';
$item_output = $args->before;
$item_output .= '<a'. $attributes .'>';
$item_output .= $args->link_before . apply_filters( 'the_title', $item->title, $item->ID ) . $args->link_after;
$item_output .= '<br /><span class="sub">' . $item->description . '</span>';
$item_output .= '</a>';
$item_output .= $args->after;
$output .= apply_filters( 'walker_nav_menu_start_el', $item_output, $item, $depth, $args );
}
}
and than locate your wp_nav_menu and update it with
<?php $walker = new Menu_With_Description; ?>
<?php wp_nav_menu( array( 'theme_location' => 'primary', 'menu_class' => 'nav-menu', 'walker' => $walker ) ); ?>