When you order a license at RedHat, you need to provide an e-mail address. In my case this was the one of the CIO of my customer. The next thing that happens is that the reseller (my company) receives an e-mail with the confirmation of the purchase and the customer receives an email with a link.
It is very important that the customer clicks this link and fills out the form. During this registration he must choose a customer name and password. Once the account is created you have to run the rhn_register command as root.
This takes you through a script where your server connects to the RHN asks for your customer name and password and gathers information about your system. Once your system is registered you can use the yum package manager to actually upgrade the system.