Well you can just load .PLN files on the world map, and start a flight. And the flight plan should be loaded.
Personally though, I don’t really like using SimBrief. It’s too much tool requiring to much input to have a proper flight plan, while we can achieve the same thing using the default world map flight planning tool which is a lot easier in my opinion. Click departing gate, click arriving gate, switch to IFR High Airways, Pick SID, pick STAR, and click Fly, and everything is loaded and ready to go.
Just make sure the Navigraph navdata AIRAC cycle is updated regularly using the Navigraph Client Beta installer. Everything should work just fine. Been doing this method for months.