hi, I had the same problem: you just have to register to
www.myarchicad.com, you can also do it by entering your previous serial number, and you will get a personal page where is shown a new serial number that would be valid for Archicad 13, but you can use it for archicad 12 too if associated with the user ID shown in that personal page, which is the e-mail address given for the registration. So if you use the old user ID it won't work, and I think you must install archicad 12 again, like I did, to change the user name.