To get your beta code you need to get your pre-order
box. When you pre-order you should recieve a box that's almost identical to a retail box for any other game, there's no game inside, but your beta passkey is. If the store you pre-ordered from doesn't carry such a box, then take your pre-order back, get a refund, and go to a store that does carry that all-important box (which, as I understand, ANet provides for them at little to no cost so tell that store exactly why they just lost your future buisiness, if you're so inclined...). There's a list on the official site of stores that should carry the proper preorder
here, although, as it lists Hastings you might want to call ahead and make sure they have the "Starter Kit" or whatever it's actually called...