Instructions
Navigate to the pre-flight briefing menu. This menu is available directly before you load your flight level after you have chosen an area and a plane to fly.
Click the "Briefing" menu from the top toolbar. A list of options will expand.
Mouse over the "Change Flight Plan" option and click "Import." A standard Windows explorer window will appear. All of your available flight plans will be automatically shown.
Click on the desired flight plan and click "Import" at the bottom-right of the window. The plan will be loaded during the level loading process.