When you launch the program from the Welcome Screen, a screen should ask for the serial number and your Adobe login.
What operating system are you working on?
If you have Premiere Elements 12 Windows, when you start the installation of the program (from installation files or disc), before the installation process goes anywhere, you are asked for serial number and country, among other things. Later on, when the program is installed, the program restarted, a Sign In dialog should appear requiring you to enter your Adobe ID and Password.
My blog post goes into a step by step of Premiere Elements 12 Windows installation
Where in that process are you?
I am not sure where your activation code comes into this.
Did you purchase Premiere Elements 12 as a purchased download from Adobe or as a purchased boxed product from an authorized reseller?
Please review and supply further details.