In the time it took to write that could you not just have helped the guy by giving him an answer instead?
If you are using simbrief install simbrief downloader and enable the options in the image below. This will add the flight plan files from simbrief automatically to your PMDG folder.
The manual address will be something like
C:\Users\Col\AppData\Local\Packages\Microsoft.FlightSimulator_8wekyb3d8bbwe\LocalState\packages\pmdg-aircraft-738\work\Flightplans
Where it says C:\Users\Col - change Col to your local username. You might need to enable view all files and folders if you are having difficulty locating the folder manually.
Hope this helps.