I load the dot pln file from the aircraft page, and then go to the dropdown menu as @CaptClarence125 suggests. It seems to work for me every time I’ve tried it.
I use Little Navmap to create my flight plan and export it to the directory that Flight Sim points to called Missions. Little Navmap knows right where to save the file…
Check out this thread if you’re interested in learning more about Little Navmap… it’s great fun!