I get that too, if I click to edit menu, or content (but not theme). Seems odd, as I thought the table would be called ps3_categories as per table in database (?) - maybe the prefix is accounted for elsewhere.
You could always edit the theme as normal, and add any static links directly in code-view of menu.php.
<li><span><a href="<?php echo($assumedRoot); ?>contactus.php">Contact Us</a></span></li>
<li><span><a href="<?php echo($assumedRoot); ?>mynewpage.php">New Link</a></span></li>
</ul>
Although you would have to reduce the width of the top level menu item in your menu.css 'ul#cssmw_home > li > span > a {' rules if you were adding a top level link, otherwise the menu wouldn't fit in the navigation div. Menu sub-items would be ok though.