Need help activating photoshop cs5 on second machine (mac)

Hello i have adobe photoshop cs5 on my windows desktop and had installed it today on my laptop which would be my second machine, i download and get to the serial number part which i referred back to a email that activated it and put that in. Each time i try to put it in i get a message saying "that code is not valid for this product" now i noticed it said cs5.1 is that any different then just cs5?

Any help is much appreciated.