I donīt think its a special option for which you pay extra, if you have bought the enterprise edition its included. (it should be, considering what it costs)
If you want to upgrade your standard edition to a enterprise edition with the same version all you have to do is use the installer and it should guide you through the process. I havenīt actually done it but since standard and enterprise is exactly the same besides the fact that enterprise contains a lot more options it should not be a problem. But read the installation instructions first to make sure. And donīt forget to backup the database first.