Please mention Music Maker version then it will be much easier to guide you.
If you are using Music Maker 16 Premium when Music Maker is open Click on "Y" key then click on Folder tab and click on Add vst plugin path then browse and locate other syth with dll file extenstion or name and add one by one.
If you are using Music Maker 16 open it and click on Add button borwse and again choose synth with dll extension or name and add one by one.