If you are using a free email address, it may take time for the registration confirmation mail to arrive. Email may also have been automatically filtered out as a spam. 

Email will be sent from "info@niconico.com" and if you have set your email service to reject mails, please change the settings so that the you can receive mails from the domain "info@niconico.com". If you have used the wrong mail address for your account please redo the registration process.