I presume that you have the basic version and not the Premium version, correct? It will say Premium if you have it.
I don't know if Music Editor comes with the basic version. If it does, you will find it under Windows, Magix, Magix Music Maker. See this thread for help with that.
If not, then you probably need a separate program. Did you read through the manual? Do you have Music Studio as well? It often comes with Music Maker.