Problems installing PE 4.0 in Mac, Mountain Lion

Since I have CS 4 running on my McBook (Snow Leopard) I don't need PE 4.0 anymore. I wanted to install Photoshop Elements 4.0 on my new Mac (Mountain Lion). When I tried, it ran through all of the necessary installation steps with one exception. I wasn't asked to type in the product key. When everything was installed I got the message: "Due to an unknown error installation failed". I don't understand why. The programme was legally purchased when I bought the old Mac. Why shouldn't I be able to install it on another computer?