I assume you want (only) the sub categories listed when on a parent category
To do this,
for the left sidebar, in header.php, line 891, find
HTML Code:
<td id="left">
or for the right sidebar, in footer.php, line 66, find
HTML Code:
<td id="right">
and add the following code below it
PHP Code:
<?php if ( is_category() ) {
$current_cat_id = get_query_var('cat'); ?>
<div class="widget"><div class="widget-title">
<h3>YOUR WIDGET TITLE</h3></div>
<div class="widget-content">
<ul><?php wp_list_categories('title_li=&child_of=' . $current_cat_id); ?></ul>
</div></div>
<?php } ?>
You could leave out this part if you want no widget title
PHP Code:
<div class="widget-title">
<h3>YOUR WIDGET TITLE</h3></div>