gistfile1.html

<style>
.hiding{display:none;}
</style>

<fb:dialog id="my_dialog" cancel_button=1>
  <fb:dialog-title>My Little Dialog</fb:dialog-title> 
  <fb:dialog-content>
    <div id='my_hidden_div' class='hiding'>hidden stuff</div>
    <div id='test'><a href='#' onclick="document.getElementById('my_hidden_div').toggleClassName('hiding')">toggle the hidden div!</a></div>
  </fb:dialog-content> 
</fb:dialog> 

<a href="#" clicktoshowdialog="my_dialog">show dialog</a>