After logging into the website"www.me.com", Go to the Account icon. It is at the top webpage, it looks like outline/shadow of a persons head and shoulders.
Then on the left side of the window click on "Account Options". In the "Account Options" pane you should see "Upgrades" and right below that "Family Pack" It should say something like 4 Unassigned and then a Blue link that says "(Set Up Account...)". Just click that.