In Magix Music Maker 16 Premium it is easy though in Magix Music Maker 16 a little bit diffiuclt, but I had used therefore, can guide you a little bit I guess.
First of all no acivation is needed all you should know is, when Magix Music Maker is open, it should not be in easy mode. Where is Easy mode? look at top left on Magix screen before File there MAGIX logo under it there are two small square click on the right side it will turn in to blue then you can see other items.
Now if you knew about easy mode then good. If you use windows xp it is easy, all VST are in Synth folder, path could be C:\Document and settings\Allusers\Music Maker_16\Synth
Click on My Computer then:
Documents and settings
All users
Application Data
Magix
Music Maker16
Synth
Here you can see all synthesizer/vst plugin
when you know the path of vst folder from Magix Music Maker click on Add button a Synth folder will be opened file with dll extension is vst plugin. This Add button is at the right side of Track synthesizer
I am using MMM 16 Premium I hope I am not wrong you could also try by File Manager.