For those who pre-ordered before December 18th, Blizzard has already invited you in, but later pre-orders will be added in waves over the coming weeks. Only select retailers offer beta keys.
Sorry but not every brick and mortar store will be able to afford you a spot in the beta if you pre-ordered with them. Amazon is one such retailer that does.
StarCraft 2: Heart of the Swarm releases on PC March 12th, 2013.