First, for accuracy, there is no such program as Music Maker Pro. There is a core free program, and Plus and Premium paid-for editions which each add various extra features.
However, you simply need to access, on your desktop, the core installer for Music Maker (perhaps the easiest method is to download the Free version from the Magix site) run the installer and then, once installed, simply enter the serial number for whichever paid-for edition you have and online register and activate it from within the program.