I hope this will not bee too confusing.

If you are talking about the B.free Internet Welcome Package there is a step-by-step guide on how to activate it and add extra packages.

Loading Money to your new Number is done via buying a coupon at Post, Supermarkets, on several cashpoints or online with credit-card. See here

After loading the money you can activate the extra packages. See Link of the step-by-step-guide under the 4th Point "Tarif aktivieren" - mehr Infos

For me it looks like that you won't need to register for an account for that. Hope that helps a bit.