If you're using Peak 3 under OS 8.6-9.x and can't open any files, try rebuilding the desktop on your computer. You can do this by restarting the computer and holding down Option+Apple when you see the extensions starting to load across the bottom of the screen. Continue holding these down until the computer prompts you asking if you're sure you'd like to rebuild the desktop.  Choose "yes" and after the desktop has finished rebuilding you should be able to open files in Peak.