To use wp_editor() you’ll need to replace your textarea tag with the output from wp_editor() like so:
case 'textarea':
wp_editor($meta, $field['id']);
echo '<br /><span class="description">'.$field['desc'].'</span>';
break;
You don’t need to echo wp_editor() since it does it automatically. You can pass an array of settings as a 3rd argument to configure it’s behaviour. More info here: