Were do you install a *.pln file in MSFS 2020?
I believe you just double-click it while the game is running and it automatically loads. I could be wrong.
whereever you want.
In the world map you clock “save/load” (at the bottom of the screen) and a pop windows (File-Dialog) will open where you can navigate to your file.
Default directory to place *.pln or *.flt files should be %AppData%\Roaming\Microsoft Flight Simulator.
At least for the Steam version.
Easy way,
start flight, hit ESC and click Load/Save. then you see on modal window your path and place 
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