When you originally went to the Microsoft site and signed up to download the
Beta-2, it would have ended up giving you instructions on how to download
the image. On that instructional screen, I believe it displayed the Key code
issued to you. At that time you should have taken the time to write it down,
but many people didn't think to do it.
At the same time Microsoft should have sent you an E-mail that would also
have included the code issued to you. You should have taken the time to
write it down at that time as well. Again many people didn't think to do
that either.
I have heard from a few people that the E-Mail from Microsoft was detected
falsely as Spam and they trashed it without looking at it.
Some people got the E-Mail from Microsoft and did take the time to print the
E-Mail for safekeeping, but forgot to check their printout to make sure it
printed properly (The code is displayed inside a yellow box, but when
printed, just the yellow box prints) and just deleted the E-Mail.
If you had taken the choice to order the DVD's (which will insure that you
automatically get the next release (RC1) shipped on DVD), then when you
recieve the DVD's there will be a code key included to be used with them.
Other than that, you may just be out of luck. Sorry.
The sign-up period for the Beta is now closed, so you can't go through the
process again to get another key from Microsoft.