Themes designed for WordPress.com blogs can only be used on WordPress.com. They cannot be used on any blog with a Cpanel, because those are WordPress.ORG blogs and they don’t use the same kind of software.
You cannot use themes downloaded from the internet on WordPress.com. The only themes we can use are found in the Theme Showcase (or from your dashboard under “appearance”). http://wordpress.com/themes/
Some may have the same name as those designed for WordPress.org, but they have been especially adapted to work with WordPress.com.
As for getting a refund from Themeforest, you’ll have to ask them what their policy is.