I just started with zen-cart too! And looking to do a custom side box so you're note popped up on my search. I don't know much zen, but the code here looks like you just have a PHP formatting problem - you need to have a valid PHP string. So to create the string you want correctly in PHP:
$content = '<div id="navmain"> '; // notes: No period before the equals before you start your content string (if you want to start it fresh). This, btw is a PHP comment
$content .= '<ul>'; // Semicolon after each statement and in each of your following statements
$content .= '<li id="nav1"><a href="index.php">home</a></li>';
$content .= '<li id="nav2"><a href="how.php">how does it work?</a></li>';
$content .= '<li id="nav3"><a href="what.php">what''s in it?</a></li>';
$content .= '<li id="nav4"><a href="faq.php">product FAQ</a></li>';
$content .= '<li id="nav5"><a href="order.php">order now</a></li>';
$content .= '</ul>';
$content .= '</div>';
Or, another way - using the HERE doc syntax (http://us3.php.net/manual/en/language.types.string.php%29:
$content = <<<EOS
<div id="navmain">
<ul>
<li id="nav1"><a href="index.php">home</a></li>
<li id="nav2"><a href="how.php">how does it work?</a></li>
<li id="nav3"><a href="what.php">what''s in it?</a></li>
<li id="nav4"><a href="faq.php">product FAQ</a></li>
<li id="nav5"><a href="order.php">order now</a></li>
</ul>
</div>
EOS;
Good luck w/ the side box and thanks for posting your experience with it as I have learned something before I add my blank sidebox!