The best bet is to go to Simroutes and select your flightplan there. Once you have your flightplan, download it in FSX format so that you can load the flightplan.
Actually, this was discussed in some detail just a month or so ago...here is a cut and paste of the technique from that thread...
Quote:
Once you have selected a flight plan, you will see a download button. There is also a drill down selection there. Click it and select FS2006(FSX). It will prompt you to accept a download. Download that file and put it in your .../My Documents/Flight Simulator X Files folder.
Next, start up FSX and select Flight Planner and then the Load button. You should see your downloaded flightplan. Select your flightplan and you should be good to go.
Make sure when you put the flightplan into your Flight Simulator X folder that you remember the name of the file and also that FSX is not running. If you place the file into the folder while FSX is running, you will have to restart to be able to select it.
You are somewhat limited with what you can do with the internal FSX flight planner. This is a workaround that will let you get a more complex plan to load.
