You log into the website with your old GW1 information (email/pass).
Once you log in with that account, then you just add the CD Key to that account, and it'll automatically be linked. If you already added your CD key to an account then you're pooped.
EDIT: Also, if you still can't get into the game, try rebooting your PC. It's passed 3PM EST which was the announced time the beta weekend event started.