Open up /theme_options/portfolio.php and change this:
$slugRule = get_option('category_base');
if($slugRule == "") $slugRule = 'category';
$args = array(
'labels' => $labels,
'public' => true,
'show_ui' => true,
'capability_type' => 'post',
'hierarchical' => false,
'rewrite' => array('slug'=>$slugRule.'/portfolio','with_front'=>true),
'query_var' => true,
'show_in_nav_menus'=> false,
'menu_position' => 5,
'supports' => array('title','thumbnail','excerpt','editor','comments')
);
to this:
//$slugRule = get_option('category_base');
//if($slugRule == "") $slugRule = 'category';
$slugRule ="";
$args = array(
'labels' => $labels,
'public' => true,
'show_ui' => true,
'capability_type' => 'post',
'hierarchical' => false,
'rewrite' => array('slug'=>$slugRule.'','with_front'=>true),
'query_var' => true,
'show_in_nav_menus'=> false,
'menu_position' => 5,
'supports' => array('title','thumbnail','excerpt','editor','comments')
);
And reset your permalinks by saving them again.