I think that's just a promo code for special avatars on SE's member page. I haven't been able to enter it either, I have signed up for the beta by putting in my computer specs and all that after creating an SE account, but I've never used a promo code for the beta.

I am off to college in the fall so I don't want to freak out over a beta I might not get into, or have the time to play if I did.