Music Maker Free does not require an activation code to be used.
You can simply ignore the activation dialogue screen and start working on a project.
If you own older Music Maker versions (P2 or P3) or the latest commercial version, then you also have the option to unlock these editions.
This will allow you to use older content, soundpools and instruments in this new version.
Click on "Magix Account".
Now enter your serial number and click on "Activate".
Log in to the store with your Magix account.