There isn't one for digital DL. Double check to make sure the game is listed as active on your account.
When installing, you SHOULD have an option to bypass registration. Something along the lines of "I've already registered." If you don't, check the tech support forums for help.
If you are installing the game right now then it already activated to your account, meaning if you can go to your account and it has diablo 3 listed as one of your games then its activated already. The game key is for people who bought disc.
Threats of violence. We take these seriously and will alert the proper authorities.
Posts containing personal information about other players. This includes physical addresses, e-mail addresses, phone numbers, and inappropriate photos and/or videos.
Harassing or discriminatory language. This will not be tolerated.