Can't install updates to my laptop

I have photoshop 12 on my desktop. I am visiting my daughter and want to install it on my laptop so I can work while I am there. I understand that you can have the program on a second computer like a laptop as long as you don't use both computers at the same time. I have photoshop on the laptop, but when I try to download the update, it says that it fails because there is no record of my basic program. I put in the serial number etc. I need to get this done. I have spent nearly all day trying to get this thing to worl. Can anyone help me? Please no shots in the dark...lolol.