I have no idea what the "$10 a year plan" is. I presume you've purchased an upgrade of some kind, but which one?
Regardless of which upgrade you've purchased, you cannot add external WordPress themes to a WordPress.com blog. If you know how to use CSS you can adapt existing external themes, but it's like translating a book. You have to be fluent in the language of CSS to do it. If you do not already know your way around CSS, you are not advised to purchase the upgrade, because there's precious little help available from volunteers and none at all from staff.