If you tried all of the above, but still cannot confirm your email:
Wait a few days, then click the 'Resend confirmation' link in the yellow nag at the top of your Twitter home page again. We send the emails immediately when you create your account, but it is possible that your email provider was temporarily blocking emails from us or delivery was delayed.
Lastly, try changing your account email, and use an email from a large domain (Gmail, Yahoo, Hotmail, etc.).