I unpack 3rd party VST's to a folder called VST which I created in C:\ProgramData\Magix\Common\ - and then add this location to MMM.
However you could unpack them almost anywhere so long as MMM knows where they are either by adding the VST folder path in the programs settings or using the Add button on the Instruments tab.
Just out of curiosity, I can't access Program Data, I have tried before. This is my PC and I am the only one who uses it but it was bought used from a dealer and I am wondering if he has set it so Program Data cannot be accessed. Or am I just showing my ignorance?
For C:\ProgramData\.... it may be hidden. Open Windows Explorer, Tools, click on the tab View, scroll down to Hidden files and folders, and change the selection to "Show hidden files, folders, and drives. Then you should see it.
For VST's, Magix created the following folder on my computer:
C:\Program Files (x86)\VSTPlugins\MAGIX and puts some of its VST's there. I simply add subfolders to the root like:
C:\Program Files (x86)\VSTPlugins\DSK
C:\Program Files (x86)\VSTPlugins\ whatever
and put all VST's in these. Magix programs scan C:\Program Files (x86)\VSTPlugins\Magix by default, so I just have programs scan C:\Program Files (x86)\VSTPlugins instead and everything is covered.
The above are for 32 bit programs. For 64 bit programs they are under:
C:\Program Files\
and since I have Samplitude, which is 64 bits, I have VST's under: