I use MMM 16 Premium and I will guide according to that may be that work in MMM MX Premium too.
First of all by external VST do you mean you have downloaded some VST Plugins and installed or you want to say VST of MMM MX Premium don't show up.
Ok, if it is the first case(You want to install external VST Plugins) you should put them (dll files) in the MMM MX Premium folder where rest of the VST are located.
How do you know where is the VST folder? in your version click on "P" it will open setting window click on Folder tab you can see path to all folders and VST. In older version of Music Maker we click on "Y" to open setting window.
Second case, if MMM MX Premium VST are not showing up. Please confirm if you are not using Music Maker in Easy mode, if so turn it off. Refer screen shot from MMM 16 Premium.