The 'computer id' is a number generated by the program you have installed when you select to register the software off-line or on another computer, if the computer you installed on to is not connected to the Internet.
In the activation/registration dialog there should be a link to register the software off-line.
If the MAC is connected to the Internet then you should be able to activate/register on-line by entering the serial number you received by email and your email address
I'm trying to install it on a new Mac. While opening, only a Reg Error window says The Activation Code is not invalid. And then nothing I can do except OK button. If click OK, than the app closes.