VST (Virtual Studio Technology) is a generic term.  It would help if you were more specific in your question.  Is this VST an instrument or an effect?  Which one?

You will place a copy of the folder (assuming there is one) containing the .dll file, and any other associated files, into the MAGIX Music Maker "Synth" folder.  A simple search of your hard drive will give you the location of the synth folder.

After you start MMM, if the VST is an instrument, you will find it in the Synth tab of the media pool.  If the VST is an effect, you will find it in the track effects slots in the mixer.

Please note, some VST plug-in's may not be compatable with MMM.