They specified this statement in the email when you buy the game.

Please log into StarCraft® II prior to launch day to begin pre-installing content for Heart of the Swarm™.

If you do not have StarCraft® II installed, please visit the Battle.net download page.

===========================================
StarCraft® II: Heart of the Swarm™ is available to play after midnight PST on 12 Mar 2013!
===========================================