First, you need to CANCEL / TERMINATE the Web Hosting Account that you want Parked.
http://www.hostingcheck.com/whmcs/knowledgebase.php?action=displayarticle&id=223
After it has been Terminated / Cancelled - Then and only then - you would login to your cPanel and Park the Domain, as shown here:
http://www.hostingcheck.com/whmcs/knowledgebase.php?action=displayarticle&id=204