all you need to do is just insert this code where you want it on the forums main page.
<div class="vbmenu_popup" id="navbar_search_menu" style="display:none">
<table cellpadding="4" cellspacing="1" border="0">
<tr>
<td class="thead">$vbphrase[search_forums]</td>
</tr>
<tr>
<td class="vbmenu_option" title="nohilite">
<form action="search.php" method="post">
<input type="hidden" name="do" value="process" />
<input type="hidden" name="showposts" value="0" />
<input type="text" class="bginput" name="query" size="20" />$gobutton<br />
</form>
</td>
</tr>
<tr>
<td class="vbmenu_option"><a href="search.php?$session[sessionurl]" accesskey="4">$vbphrase[advanced_search]</a></td>
</tr>
</table>
</div>