I would have thought Quartus 9.0SP2 was the correct version for you to use - its certainly the version I use for FLEX10K10A and FLEX10K100E device designs.
I just checked the Devices menu and can confirm that the 10K50G part is not mentioned.
I recall that you can provide a part that is not listed using the Tcl command
set_global_assignment -name DEVICE "EPF10K50RC240-3"
where in this case, I've used a valid part number. I tried '240 without the -3 speed grade and its not a valid part number, so the part number you provided is incomplete as far as this Tcl command is concerned. Try entering the full part number including speed grade using this Tcl command.
Alternatively, file a SR with Altera directly.
Cheers,
Dave