When i pasted the code you provided into Dreamweaver CS5, i got a syntax error. I had ago myself with the code you wrote and got it to work.
Here’s what my code looks like now:
<div id="navbar">
<ul class="ot-menu">
<li<?php if (is_home()) { echo " class=\"current_cat\""; }?>><a href="<?php echo get_settings('home'); ?>"><?php _e("Home", 'structuretheme'); ?></a></li>
<?php wp_list_categories('title_li=&depth=4&sort_column=menu_order'); ?>
</ul>
<div style="clear:both;"></div>
<ul id="menu">
<li <?php if(is_home()) { ?> class="current-cat"<?php } ?>>
</div>
It looks a bit messy now! If you could help me make it a bit neater, that’d be great! Most importantly, the code is now working (see https://schristian.eu).
Thanks for your help Woodent!