how do users register for a prosites site?

how do i make it so users can signup for a prosites?

i do understand the plugin correct? users can signup, and get their own blog? i set it up in my network now. i assigned themes and plugins. i assigned prices and everything (i guess everything).

so how do i manage to let them sign up, and see the dialog where they can chose from the plans i configured? when i let them sign up via the sign-up.php, the register as a new user for MY site. and thats not what i want. and thats not how i understand the plugin is supposed to work, right?
so how on earth do they get their own blog, with their own quota and the option to pay?
please. this is kind of urgent...