Open includes > loop-portfolio.php and find this code
$sort_classes = "";
$item_categories = get_the_terms( $the_id, 'portfolio_entries' );
if(is_object($item_categories) || is_array($item_categories))
foreach ($item_categories as $cat)
$sort_classes .= $cat->slug.'_sort ';
That is the code for the portfolio sortable categories. The default order is alphabetical. Not sure if you can sort it though. Let me tag the rest of the support team.