Find in your templates the set "Navigation/Breadcrumb Templates", double click it to expand, and then locate within those the template navbar. Open it up.
Then look for the following code.
<!-- nav buttons bar -->
And put under it this code, replacing the url with the one you need, and text with what you want the link to say.
<td class="vbmenu_control"><a href="www.defaulturl.com">Text</a></td>