There are several ways to do this, but the safest one is to create a new advanced theme following the instructions here:
http://www.question2answer.org/advanced.php#theme-advanced
In your theme's qa-theme.php file, override the nav_list() method to insert the link:
<?php
class qa_html_theme extends qa_html_theme_base
{
function nav_list($navigation, $navtype)
{
if ($navtype=='main')
$navigation['custom']=array( 'url' => 'http://your-page/', 'label' => 'your-label');
qa_html_theme_base::nav_list($navigation, $navtype);
}
}
?>
It's planned to make this much easier to do in future, via the admin interface.