Jorang,
you need to include the file before you use it. To get started just do this.
$form_data = array('DATA_1' => $variable, 'DATA_2' => $variable2);
$a = new http_post;
$a->set_action('http://www.whatever.com/cgi/act.php');
$a->set_timeout(30);
$a->set_element($form_data);
if ($debug) $school->show_post();
$content = $school->send($debug) or die("http_post's send failed!");
echo $content;
The rest of the settings and instructions are in the http_post file, most of them are defaulted.
GL