I downloaded InStat Mac (or Prism Mac or StatMate Mac), but when I double click on the installer, classic (OS9) mode is launched. I want to run InStat for OSX without classic mode.

Our Mac programs come in only one version that will install and run on either OS9 or OSX. Somehow, your Mac is confused and is launching the installer in Classic mode.

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. Another way to do this is to locate the installer file, click once to select it, and then choose Get Info from the Finder's File menu. Now make sure that checkbox "Open in the Classic environment" is not checked.