I cannot install the Prism Mac (or InStat) Demo version because the installation program requests the classic environment OS 9 even though I am running OS X.

All our demos are available in two formats for the Mac. Make sure you download the .IMG format, which works well on OSX, rather than the .SIT format.

You can tell your Mac to temporary turn off the Classic mode. Do this in System Preferences, Classic. With classic mode turned off, the GraphPad installer ought to launch in OSX.