Just thought I'd post on here, but the beta code "choosing" is now live on Bungie.net. You must select which platform you wish to download the beta from. They just opened this today, but did not specify how long it will be going for (presumably til Beta ends, but who knows). I'd get there now and select whichever platform you intend on getting the game on before they "run out". You can also redeem the codes on my 1st post here while you're at it if you haven't yet done so.
http://www.bungie.net/en/User/redeemedcodes