Photoshop Elements 6 requires a PowerPC processor or at least Rosetta. Do you have a PowerPC processor or is it an Intel based processor? You can find out details about your system by going to the Apple Menu>About This Mac.
You will need to install Rosetta to possibly have Photoshop Elements function. Please see If you need to install Rosetta - http://docs.info.apple.com/article.html?path=Mac/10.6/en/27987.html for details on installing Rosetta.