in the navbar template located in
Styles & Templates > <Style> > Navigation & Breadcrumb Templates > Navbar
Where <style> = the style you'd like the code in,if you have multiple styles, you'll need to add it to each unless they are a childstyle.
If your site rules are a thread, then You'll have to link to the thread. But basically it would look like this
<td class="vbmenu_control><a href=$vboptions[bburl]/showthread.php?t=xxxx">SiteRules</a></td>
Add that anywhere you'd like. Or instead of "Site Rules" you can create a custom phrase and use $vbphrase[phrasename].
If your site rules are on a separate page, just replace the web address andhard code the addy instead of $vboption[bburl] or just use "/showthread.php"
You don't really need the $vboptions[bburl] but i usually add it anyways. That's just me.