Crie arquivo bootstrap_menu.php

<?php
class twitter_bootstrap_nav_walker extends Walker_Nav_Menu {

function start_lvl( &$output, $depth ) {
$indent = str_repeat( "\t", $depth );
$output    .= "\n$indent<ul class=\"dropdown-menu\">\n";
}

function start_el( &$output, $item, $depth = 0, $args = array(), $id = 0 ) {
global $wp_query;
$indent = ( $depth ) ? str_repeat( "\t", $depth ) : '';

if (strcasecmp($item->title, 'divider')) {
$class_names = $value = '';
$classes = empty( $item->classes ) ? array() : (array) $item->classes;
$classes[] = ($item->current) ? 'active' : '';
$classes[] = 'menu-item-' . $item->ID;
$class_names = join( ' ', apply_filters( 'nav_menu_css_class', array_filter( $classes ), $item, $args ) );

if ($args->has_children && $depth > 0) {
$class_names .= ' dropdown-submenu';
} else if($args->has_children && $depth === 0) {
$class_names .= ' dropdown';
}

$class_names = $class_names ? ' class="' . esc_attr( $class_names ) . '"' : '';

$id = apply_filters( 'nav_menu_item_id', 'menu-item-'. $item->ID, $item, $args );
$id = $id ? ' id="' . esc_attr( $id ) . '"' : '';

$output .= $indent . '<li' . $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 ) .'"' : '';
$attributes .= ($args->has_children) ? ' data-toggle="dropdown" data-target="#" class="dropdown-toggle"' : '';

$item_output = $args->before;
$item_output .= '<a'. $attributes .'>';
$item_output .= $args->link_before . apply_filters( 'the_title', $item->title, $item->ID ) . $args->link_after;
$item_output .= ($args->has_children && $depth == 0) ? ' <span class="caret"></span></a>' : '</a>';
$item_output .= $args->after;

$output .= apply_filters( 'walker_nav_menu_start_el', $item_output, $item, $depth, $args );
} else {
$output .= $indent . '<li class="divider">';
}
}

function display_element( $element, &$children_elements, $max_depth, $depth=0, $args, &$output ) {

if ( !$element ) {
return;
}

$id_field = $this->db_fields['id'];

if ( is_array( $args[0] ) )
$args[0]['has_children'] = ! empty( $children_elements[$element->$id_field] );
else if ( is_object( $args[0] ) )
$args[0]->has_children = ! empty( $children_elements[$element->$id_field] );
$cb_args = array_merge( array(&$output, $element, $depth), $args);
call_user_func_array(array(&$this, 'start_el'), $cb_args);

$id = $element->$id_field;

if ( ($max_depth == 0 || $max_depth > $depth+1 ) && isset( $children_elements[$id]) ) {

foreach( $children_elements[ $id ] as $child ){

if ( !isset($newlevel) ) {
$newlevel = true;

$cb_args = array_merge( array(&$output, $depth), $args);
call_user_func_array(array(&$this, 'start_lvl'), $cb_args);
}
$this->display_element( $child, $children_elements, $max_depth, $depth + 1, $args, $output );
}
unset( $children_elements[ $id ] );
}

if ( isset($newlevel) && $newlevel ){

$cb_args = array_merge( array(&$output, $depth), $args);
call_user_func_array(array(&$this, 'end_lvl'), $cb_args);
}
$cb_args = array_merge( array(&$output, $element, $depth), $args);
call_user_func_array(array(&$this, 'end_el'), $cb_args);
}
}
?>

Arquivo menu.php

<div class="navbar">                                            <!-- Div Navbar -->
   <button data-target=".nav-collapse" data-toggle="collapse" class="btn btn-navbar" type="button">
      <span class="icon-bar"></span>
      <span class="icon-bar"></span>
      <span class="icon-bar"></span>
   </button>
   <div class="navbar-inner"> <!-- Navbar-inner -->
      <div class="nav-collapse collapse"> <!-- Nav-collapse -->
         <nav class="menu"> <!-- Main nav -->
            <?php
            wp_nav_menu(array(
              'container' => false,
              'items_wrap' => '<ul id="%1$s" class="%2$s nav">%3$s</ul>',
              'walker' => new twitter_bootstrap_nav_walker
            ));
            ?>
         </nav> <!-- End Nav Tag -->
      </div> <!-- End Nav-collapse -->
   </div> <!-- End Navbar-inner -->
</div>

Snippet de chamada do Menu no head ou qualquer outra parte do Tema

<?php get_template_part('menu') ?>