Let me explain it a little bit better.
It is impossible to open a file created on CS(something) on macromedia flash 8, if you have a CS5 file, you need someone with flash CS4 and flash CS3.
Then open flash CS5, and convert it down to CS4, then the person who has CS4 will convert it down to CS3, and finally the guy who has CS3 will convert it down to flash 8, and only THEN you can open the file.
That's the only solution. Also you CAN open a flash 8 file on any other version, you just can't do it backwards.