Starting June 28th, you will no longer be able to log in to a Curse account that was not merged with a Twitch account. If you have not yet merged your Curse account with a Twitch account, please do so here! Otherwise, your account and its content will be inaccessible.
Since you purchased this digitally you would not receive a key, it is just added to your account. Looking at your account you should be good, if its not showing up just completely exit Battle.net and log back in and then you should be golden.