It's just a link. Edit the SHOWTHREAD template, search for advanced_search to find the part below, then add the code in red above it:
Code:
<tr>
<td class="vbmenu_option"><a href="search.php?$session[sessionurl]searchthreadid=$threadid&userid=$bbuserinfo[userid]&do=finduser">Find My Posts</a></td>
</tr>
<tr>
<td class="vbmenu_option"><a href="search.php?$session[sessionurl]searchthreadid=$threadid">$vbphrase[advanced_search]</a></td>
</tr>
(if you want a phrase for the text, you'll have to figure that out yourself

)