It can't work, your code is shitty!
Your code:
<div id="header">
<div id="menu">
<!--<li class="pagenav"><h2>Pages</h2><ul><li class="page_item page-item-2"><a href="http://stigr-lighting.com/?page_id=2" title="About">About</a></li>
</ul></li>-->
<li>
<ul><a href="http://stigr-lighting.com/wp-content/themes/default/about.php">About</a></ul>
<ul><a href="http://stigr-lighting.com/wp-content/themes/default/about.php">Gallery</a></ul>
<ul><a href="../../cv.php">CV</a></ul>
<ul><a href="../../links.php">Links</a></ul>
</li>
</div>
</div>
should look like:
<div id="header">
<div id="menu">
<li class="page_item page-item-2>Page1</li>
<li class="page_item page-item-2>Page2</li>
<li class="page_item page-item-2>Page3</li>
.
.
.
</div>
</div>
<hr />
So clean your HTML to something like this:
in your header.php:
<div id="header">
<div id="menu">
<?php wp_list_pages('title_li='); ?>
</div>
</div>
<hr />
and in your style.css:
#menu li .page_item {display:inline;}