Add this to your categories.html template:
{whos_online}
Open categories.php and change this:
$templates_used = 'categories,category_bit,thumbnail_bit';
$main_template = 'categories';
define('GET_CACHES', 1);
define('ROOT_PATH', './');
To this:
$templates_used = 'categories,category_bit,thumbnail_bit,whos_online';
$main_template = 'categories';
define('GET_CACHES', 1);
define('ROOT_PATH', './');
define('GET_USER_ONLINE', 1);