You will need to save the file as a CS5 file if you want to open it in CS5. I do not know if CS6 saves for CS5 but it is likely that it does since Flash normally allows saving one full version back. Choose Save As when you go to save it and select CS5 as the file type/version.
Flash Pro CS6 supports saving as a Flash CS5.5 or Flash CS5 documents(fla and XFLs).
You can save this to either of these formats using the File->Save As option