Depending on the package you buy it in, you will be given a card that explains how to obtain the 64bit version. In short, they charge you an additional fee to get what should be offered on store shelves. After you pay, they mail you the disk you requested (32bit or 64bit).
I'm a bit ticked, having spent money on Home Premium, only to find I had to pay MS an additional 10$ to order the 64bit version. On top of that it takes up to 10 days to arrive, 16$ if you get express shipping.