You'll edit the navbar template. I'll give you a sample code to start off with and then just make the adjustments as needed:
Find:
<if condition="$show['registerbutton']">
And before that code add:
<td id="samplemenu" class="vbmenu_control"><a href="$show[nojs_link]#samplemenu">$vbphrase[quick_links]</a> <script type="text/javascript"> vbmenu_register("samplemenu"); </script></td>
Now find (at the very near bottom of navbar template):
<!-- / NAVBAR POPUP MENUS -->
And add before:
<!-- samplemenu menu -->
<div class="vbmenu_popup" id="samplemenu_menu" style="display:none">
<table cellpadding="4" cellspacing="1" border="0">
<tr><td class="thead">My Samplemenu Test</td></tr>
<tr><td class="vbmenu_option"><a href="#">Link 1</a></td></tr>
<tr><td class="vbmenu_option"><a href="#">Link 2</a></td></tr>
<tr><td class="vbmenu_option"><a href="#">Link 3</a></td></tr>
<tr><td class="thead">Another Category Here</td></tr>
<tr><td class="vbmenu_option"><a href="#">Link 4</a></td></tr>
</table>
</div>
<!-- / samplemenu menu -->
Save and your test dropdown menu will appear next to the User CP link. :)
You can change the ID of "samplemenu" to whatever you want. Just make sure that you update that change everywhere that is in red. You'll also need to add the URLs in the "a href" tag and change the links and headers.
I hope that helps. :)