FSC8 saves flight plans in a dual format (in the same PLN file) - the old FS9 format and it's own format! Although FSX can read and understand the FS9 format included in the PLN file, the CS FMC can't.

FSC9.x creates flight plans in FSX flight plan format (as well as many others), saving it as an FSX flight plan, it saves 2 different formats (in two different files) - FSX flight plan format (PLN) in the Documents\Flight Simulator X files folder, and it's own format (also PLN, but in a different format inside it, not FSX FP format) in <FSC9.x root>\Flightplan\FSC folder.

So the CS 777 FMC can load the flight plans created with FSC9.x.