This is because you have an older version of Stuffit. Download and install the latest version (10) from www.stuffit.com and the .zip file will open properly. It will prompt you to load your disks one at a time and install the large data file (.dat). You will then need to download and install the appropriate plug-in update (AU, VST, and/or RTAS) from www.spectrasonics.net/updates. Place the .dat file, or an alias of it, in the appropriate plug-ins folder.