I just checked and when i do it (enter phone number, select device) via 'sign up' or 'get started' then press 'next' it takes me to page 2 which is where I enter my email details. Not sure why it is not doing it for you. I actually just set mine up a few days ago.

Nokia messaging is on your device ( I dont have mine yet) so did you try setting up from there? When you first set up your username and password is actually usually your main email address and the passwordf or that.