Code:
<li<?php if(is_category('tutorials') || in_category('tutorials')) { echo ' class="current"'; } ?>><a href="<?php get_option('home'); ?>/tutorials/">Tutorials</a></li>
Sorta thing.
Basically that adds ' class="current"' to the <li>, if the user is viewing the category archive for 'Tutorials', or the post they're viewing is in the 'Tutorials' category.