Hmm... it didn't work. I added it like this:
// The [calendar]!
echo '<li><a class="calendar' , $current_action == 'calendar' ? ' active' : '', '" href="', $scripturl, '?action=calendar">' , $txt['calendar24'] , '</a></li>';
// The [shop] button
echo '<li><a class="shop' , $current_action == 'shop' ? ' active' : '', '" href="', $scripturl, '?action=shop">' , $txt['shop'] , '</a></li>';