Download the beta client if you ordered before Dec. 18
Ahead of the full release this March, StarCraft II: Heart of the Swarm has expanded its beta to include people who pre-purchased a digital copy of the expansion on Battle.net on or before December 18. If that describes you, be sure to download the client and start playing.
According to Blizzard, those who pre-purchase after that date aren't completely out of luck -- further waves of these players will be given beta access "on an approximately weekly basis." The company also reminds that participating retailers, such as Amazon, have beta keys.