Assuming you have a fully operational set-up (GS/MU or just MU), you can now manually “Mlat” non ADS-B aircraft. For the most part these will be military assets, though it will become apparent that there are a fair few civil types that don’t transmit positional data too.
Unless you have read the Help Topics within PlanePlotter or the PDF file above it may not be that obvious what to do now and how Mlat works. Simply put – if a precise distance from an unknown location (position-less aircraft) can be received by 3 known locations (Ground Stations) it is just a matter of using geometry to determine the position of the unknown aircraft. Of course it is way more complicated than this simplified explanation and I recommend reading the PDF file above for a more thorough briefing.
To use the PP Mlat feature go to View… Aircraft list as position-less aircraft obviously do not appear in the chart view until successfully plotted. It is useful to mark additional sub search parameters from the Aircraft list display options notably: Aircraft without positions (shortcut key U) or Aircraft with Mlat possible (shortcut key M) as this will eliminate aircraft that have positions and make it easier to see what is or not plotted.