It is not guaranteed that third party Vst plugins will work in older version of Magix Music Maker. Some works and some don't. Therefore, do not buy first use trial version. Even you can use Free version.
This is how to add plugin:
If there is a folder and inside that you got Vst plugin just copy the entire folder in to the Synth folder of Music maker. Or if in case you got only dll files just copy that too in the Synth folder.
Start Music Maker and the newly added plugins will be added.
Be aware that while it should be added to the Synth folder, it may not show up under the Synth tab in the media pool. If it is actually an effect, it will be found in the effects slots for each track in the MIXER.