Hello there Alan, unfortunately you can't use this theme on WordPress.com, you need a self-hosted WordPress installation for this to work. WordPress.com provides a limited selection of themes and that's it. All other premium themes are for self-hosted WordPress installations. You can see the differences here http://en.support.wordpress.com/com-vs-org/
To work with a self-hosted installation you need to buy a hosting package from a web hosting service provider and setup the installation there. I'm not sure that you will be able to use the domain you bought through WordPress.com in that case though.
I'm thinking that you have bought the template without knowing about the separation that exists in WordPress. I go out on a limb and say that you probably don't want to leave WordPress.com for a self hosted WordPress installation.
I would suggest contacting envato, explaining in short what happened, so they can refund you.