Okay so for me it works. What I did: Go on simBrief, create a flight plan. Scroll down and choose Google Earth KML as the file type. Go on the website you mentioned, upload the file. It then automatically downloads the flight plan file in the .fpl format. Then, I went into the sim, spawned in somewhere and opened the flight plan view on the map. There I clicked on the button with the 2 arrows in opposite directions and chose the file. Did you do the exact same steps?
Do you allow IF to access your files?