You will have to simply keep trying until the registration system clears up, currently it's getting so bogged down that not all registrations are going through successfully.
*Some on GW2Guru mentioned that they refreshed the error page about 10 times and it went through for them, i am not sure if that will work for everyone.
Update: Even if you keep running into the 502 error, keep checking your email you used to link the Guild wars 1 account or the e-mail you used in the registration process, you'll get an e-mail eventually regardless of whether or not you can't get past step 2 on the registration website. From there you can simply log in to the beta forums and the client download link will soon follow.
Last edited by Neoyoshi on Tue Apr 10, 2012 4:04 am; edited 1 time in total