Add TinyMCE Metabox in WordPress

gistfile1.txt

function additional_info() {
	global $post;
	$custom = get_post_custom($post->ID);
	$additional_info = $custom["additional_info"][0];
	// this adds an extra tinymce field
	?>
	<script type="text/javascript">
	jQuery(document).ready(function() {
		jQuery("#additional_info_textarea").addClass("mceEditor");
			if ( typeof( tinyMCE ) == "object" && typeof( tinyMCE.execCommand ) == "function" ) {
		tinyMCE.execCommand("mceAddControl", false, "additional_info_textarea");
		}        
	});
	</script>
    <style type="text/css">#additional_info_textarea_ifr { border:1px solid #DFDFDF; }</style>
	<p><label style="display:none;">Any additional information:</label><br />
	<textarea cols="50" rows="5" id="additional_info_textarea" name="additional_info"><?php echo $additional_info; ?></textarea></p>
<?php
}
// enable new wysiwyg features
add_filter('mce_css', 'my_editor_style');
function my_editor_style($url) {
	if ( !empty($url) )
		$url .= ',';
		// Change the path here if using sub-directory
		$url .= trailingslashit( get_stylesheet_directory_uri() ) . 'editor-style.css';
	return $url;
}