You can install the program on 2 computers, if the old computer was the only one it was installed on, then install MMM on the new computer - the download is available in your account at magix.com under My Products.
When you start MMM login to the Store - there should be no need for the serial number, and activation should complete. You can then download an install the extra content and any additional soundloops, instruments etc you have purchased.
Check the program is running correctly, then transfer your projects, exports etc from the old computer to the new if required.
You can uninstall the program from the old computer - this should also deactivate the old computer registration in you account at magix.com.
From which online supplier did you buy MM? ALL Magix products require a serial number/activation key to enable them to run. This must be entered by the user.
Pleas do not post order confirmations, serial numbers or any other personal or confidential information in the forum. This is a public forum not Magix Support
The serial number should be in the email you received confirming purchase of Music Maker - it begins with P3- followed by 5 groups of 5 digits. If you did not receive the email check your spam folder.