Once you install Poser 5, you can double-click on any pz3 file and it will open in Poser 5. Keep this in mind if you have Poser 4/ Pro Pack installed on the same machine: Poser 5 changes the registry setting for pz3 files. So, if you double-click and expect Poser 4 to open, you'll get a nice suprise when you see P5 come up.