Can?t make the activation automatically of the blog when finishing the registration process

Hi WPMU, i have follow all the steps as it described here:

I am not having success with the auto creation of the blog after the plan purchase or the free registration option. I don't know why but when i finish the purchase or select the free registration option it does not create any account with the sub-domain that i have created on registration process or even a blog or user registration account!

I have no idea why it does not create the blog account automaticaly using the subdomain that i provided.

Can you please help me with this?