Multi-Level navigation menus

Hi ~

I’m using a free theme where I want to implement the drop-down child menus in the nav bar. I’ve tried the following code to display the child pages as a drop-down but it doesn’t work.

<?php
  if($post->post_parent)
  $children = wp_list_pages("title_li=&child_of=".$post->post_parent."&echo=0");
  else
  $children = wp_list_pages("title_li=&child_of=".$post->ID."&echo=0");
  if ($children) { ?>
  <ul>
  <?php echo $children; ?>
  </ul>
  <?php } ?>

I don’t know if my CSS is the problem or maybe the PHP code. Can someone help me?