WP Math Captcha

wp-mc.php

<?php
/*
Plugin Name: WP Math Captcha
Plugin URI: http://projects.malcom.pl/plugins/wordpress/wp-mc.xhtml
Description: Simple math captcha for wordpress comments.
Version: 0.3
Author: Marcin 'Malcom' Malich
Author URI: http://malcom.pl
*/

register_activation_hook(__FILE__, 'wpmc_activate');
add_filter('preprocess_comment', 'wpmc_preprocess_comment');


if (WPLANG === 'pl_PL') {
	define('WPMC_LABEL',		'Wynik działania %s');
	define('WPMC_ERROR_MSG',	'Błąd: proszę wprowadzić poprawny wynik działania.');
} else {
	define('WPMC_LABEL',		'Result of operation %s');
	define('WPMC_ERROR_MSG',	'Error: please enter a valid result of math operation.');
}


function wpmc_activate() {

	$conf = array (
		'salt'	=> uniqid()
	);

	add_option('wpmc_count', 0);
	add_option('wpmc_config', $conf);

}

function wpmc_genhash($data) {

	$conf = (object) get_option('wpmc_config', 0);
	return sha1($conf->salt . $data);

}

function wpmc_preprocess_comment($incoming_comment) {

	if (is_user_logged_in())
		return $incoming_comment;

	// pingback, trackback
	if ($incoming_comment['comment_type'] != '')
		return $incoming_comment;

	if (wpmc_genhash($_POST['wpmc']) != $_POST['wpmch']) {
		update_option('wpmc_count', get_option('wpmc_count', 0) + 1);
		wp_die(WPMC_ERROR_MSG);
	}

	return $incoming_comment;
}

function wpmc_form_input() {

	if (is_user_logged_in())
		return;

	srand(time());
	$a = rand(1,9);
	$b = rand(1,9);
	$c = rand(1,9);

	$os = "$a * $b + $c";
	$oh = wpmc_genhash($a * $b + $c);
?>

<p>
	<input type="hidden" name="wpmch" id="wpmch" value="<?php echo $oh; ?>" />
	<input type="text" name="wpmc" id="wpmc" value="" size="22" />
	<label for="wpmc"><small><?php printf(WPMC_LABEL, $os); ?> <?php _e('(required)'); ?></small></label></p>
</p>

<?php

}

function wpmc_count() {
	echo get_option('wpmc_count', 0);
}

?>

how can use in wordpress comment form?