It is not clear to me whether you are trying to create an account on the T-Mobile Web site, or access the admin page for your hotspot hardware device.
If hardware, remember that the password for logging into the WiFi network is different from the one for the admin page. After, you wouldn't want everyone you permit to share your hotspot to also have admin privileges, would you<G>?
Did you get a "Start Guide" with your Hotspot? If so, the details for accessing the admin page are on pages 12 - 16. If not, you can find a copy at http://support.t-mobile.com/docs/DOC-1936.
Please let us know if this answered your question -- thanks!