Hello Philip,
The similar discussions module will only appear when you view a discussion item in EasyDiscuss. It then takes the title and content to match against any other discussions on the site. Unfortunately right now it's not possible to have similar items based on the category. You can however, hack the file /modules/mod_easydiscuss_similar_discussions/helper.php and at line 53 locate the codes below,
// $query .= ' AND a.`category_id` = ' . $db->Quote( $categoryId );
Replace it with,
$query .= ' AND a.`category_id` = ' . $db->Quote( $categoryId );