I just purchased HotS and had problems finding out how to download it. Blizzard said in the e-mail to "log in now to download..:" and "look at your starcraft 2 dashboard" but that didn't work. I finally got to download it and thought I would help others with the same problem.

What I finally did was simply go to blizzard's HotS beta download site and copy the link for windows users and pasted it in the adress bar of the browser and just changed the .exe to .zip and it worked perfectly...

here is the link that I used:
StarCraft-II-Beta-Setup-enUS.zip (Mac version)

I hope it will be of some help!