If you already had an account on the old forums, an account with the same user name/email has been created for you without a password. So just press the log In button and click on the I forgot my password link to set a password for your account.
The old forums are available at http://old.forum.tribler.org and will be for a while.
Enjoy the all new forums!
Update: It seems we are having some issues with the mailing system, so it may take a few hours to receive your activation e-mail. Sorry for that.
Update 2: The mail delivery system seems to be up and running now. Please, retry if you haven’t got your e-mail yet.