Follow the instructions here to add a game authentication key to your Blizzard battle.net account. Useful for any Blizzard Entertainment game including the Warcraft, Starcraft and Diablo series.
1. Go to http://us.battle.net/
Now that your game is added to your battle.net account, you can download the game client to install the game on your computer. This is helpful if you don’t want to do a lot of disc switching during install or if you don’t have the game discs at all. Follow these instructions to download the game client to your computer and start installing your game.