Don't do any manual queries:
This is kinda backwards, but the URL;
http://yoursite.com/index.php/yourmodule?confirmid=$confirmid
Replacing $confirmid with the right one, of course
<?php
$_GET['confirmid'] = $conf_id; // This will be from the URL above, you don't actually need this line
RegistrationData::ValidateConfirm(); // This is an old function, but still works
There used to be email confirmation, but that was disabled in favor of the admin confirmation