You’ll get an Origin CD Key for Rocket Arena that needs to be activated on the Origin Client. Log in and activate the key there, making it ready for download. You can do that by clicking on “Redeem Product Code” and enter the code there, and the game will be added to your games list. If you don’t have the Origin Client yet, you can download it here.
A Rocket Arena key for the Origin-Client does not expire and after activation Rocket Arena can be downloaded and played forever.