Ok, firstly you should use the modification requests or general vBulletin questions for this type of question.
Anyway, while I'm here, here is the answer.
locate your template navbar
find
<td class="vbmenu_control"><a href="faq.php$session[sessionurl_q]" accesskey="5">$vbphrase[faq]</a></td>
<td class="vbmenu_control"><a href="memberlist.php$session[sessionurl_q]">$vbphrase[members_list]</a></td>
<td class="vbmenu_control"><a href="calendar.php$session[sessionurl_q]">$vbphrase[calendar]</a></td>
replace with
<td id="custommenu" class="vbmenu_control"><a href="$show[nojs_link]#custommenu">TEXT HERE</a> <script type="text/javascript"> vbmenu_register("custommmenu"); </script></td>
find
<if condition="$show['popups']">
<!-- NAVBAR POPUP MENUS -->
add below
<div class="vbmenu_popup" id="custommenu_menu" style="display:none">
<table cellpadding="4" cellspacing="1" border="0">
<tr>
<td class="thead">TITLE</td>
</tr>
<tr><td class="vbmenu_popup"><a href="faq.php$session[sessionurl_q]" accesskey="5">$vbphrase[faq]</a></td></tr>
<tr> <td class="vbmenu_popup"><a href="memberlist.php$session[sessionurl_q]">$vbphrase[members_list]</a></td></tr>
<tr> <td class="vbmenu_popup"><a href="calendar.php$session[sessionurl_q]">$vbphrase[calendar]</a></td></tr>
</table>
</div>
follow the template to add additional links to the menu via the final edit