I just installed MachFive, MX4, MSI, or Ethno and it does not show up as a plugin.
Virtual instruments now have their own type of track, the Instrument Track.
In previous versions of DP, software synth plug-ins showed up on track inserts. In DP 4.12 and higher, to load a soft synth, go to: Project Menu > Add Track > Add Instrument Track and choose your soft synth. The synth you create will show up in the mixing board on a dedicated instrument track. Double-click on the insert to call up the plug-in window.