The easiest way is to simply login to your Battle.net account using your CD key.
If you can’t get a battle.net login, or the servers are overloaded it can be dl’d via torrent from: https://thepiratebay.org/torrent/5695409/StarCraft_2_Wings_Of_Liberty_Digital_Download-WW
You will still need a key. There are a number of places selling them, including Kmart which had SC2 as low as $20.
