[Plugin: Multiple Post Thumbnails] external link on thumbnail

Hello and thanks for a great plugin! 🙂

I’m having a problem with putting an external link on the thumbnail. I found out that this has to be done using custom fields and I even found the code that needs to be applied for this to work on a standard featured image:

<?php
 if(has_post_thumbnail()) : ?>
 <a href="<?php echo get_post_meta($post->ID, 'featuredlink', true); ?>">
  <?php the_post_thumbnail(
     array(300, 225),
     array("class" => "alignleft featured_image")
     ); ?>
  </a>
<?php endif; ?>

Then simply in the custom fields sections, I enter featuredlink in the name and the full url (including http://) into the value.

The problem is that I don’t know at all PHP and I have no idea how to do the same for this plugin. Can someone help me with this?

Thanks!

http://wordpress.org/extend/plugins/multiple-post-thumbnails/