They use the email address you give them on the Registration page to send you a note of confirmation.  You need that note to activate your account.  They also say on the Registration page that there are certain e-mail services you can't use for this as they will auto-dump the Nyaa note as spam.

Hope that helps.

Additionally NYAA confirmation links often time out, therefore if you have clicked on the confirm link option in the email or have tried to confirm using the information in the email and it's still id giving you a problem, then go to log in page and it will give you the option to resend the confirmation email. Make sure you access the email as soon as possible to prevent complications.
Good luck and let us know if any additional problems