You don't load "My_form_validation", you just load form_validation like normal. CI will internally load MY_Form_validation IF it exists.
You also still just use the "form_validation" object, like here:
$this->CI->my_form_validation->set_rules($config_rules);
Just make sure the MY_Form_validation.php file exists in /application/libraries, since it's a CI native library you are extending.
