Free level by default?

Hello,

I see in the Pro-Site admin's page that Pro Sites has a built-in free level by default.

But I don't find how to enaible it by default for every new user signup.
If I signup as new user, and I try to entre in the admin panel, I will be redirected to the page "You must pay to enable your site".

What I have to configure to allow free accounts?

Thank you for your help.
Regards
Nicolas.