First thing to try is running the PF3_Register.exe program from your main PF3 folder. Right-click on it and select 'Run as Administrator'... you won't see anything happen but if you wait about 15 seconds then try the flight plan again.
If that doesn't help please download the Microsoft DirectX for Managed Code Runtimes, using the following link:-
http://www.ocs-support.co.uk/downloads/ ... ntimes.zip
Once downloaded you need to unzip the content into a temporary folder anywhere on your system, then run the downloaded EXE. You will be prompted for a location to copy the extracted files into, so just choose the same temp folder.
Once that's done go into the temp folder and right-click the 'DXSetup.exe' program and then select 'Run as Administrator'
This will NOT alter any of your currently installed DirectX files but merely installs any missing runtimes in parallel
Once the DirectX Runtimes are installed please try running PF3 again and connecting to FS and let me know what happens. You can also delete the downloaded runtimes and temp folder if you wish.
If you still have problems after this then try uninstalling/reinstalling PF3