Xtra is in the throes of "upgrading" its email service to align itself with Yahoo. You will have heard and read that all did not go according to plan. However, members who have an Xtra account must register under the new Bubble facility - there is no option not to do so. Although currently you are unable register, due to the technical problems, Telecom say they will probably be fixed within a few days and you will then be able to join.
An important point is that if you want to access your account via Webmail you do not now click the Webmail link. This will only take you to your Yahoo account - you have to click the Bubble link - but of course this will only work once you have registered the Bubble facility.
So the new web address for Xtra is now http://nz.yahoo.com and you click the Yahoo!Xtra Bubble link not the Webmail link which will take you to Yahoo.