All the .zip files on this site are made using Mac OS X's built-in "Create Archive" feature. The .zip files this creates are not quite compatible with Stuffit Expander. I'm not sure whose fault this is, but to be sure of a successful outcome, always use the system's built-in unzipper with these files. This unzipper goes by the user-awkward name of "BOMArchiveHelper"* which is the default application mapped to the .zip extension.
It's not uncommon for .zip files to get mapped to Stuffit Expander however, so if this is the case on your system, simply double-clicking the .zip file will result in an archive that is not correctly unpacked. If the archive contains an application, very often the symptoms of the problem are that the application just won't launch - it goes through the motions but nothing happens. The other giveaway is that within the unpacked folder, you'll see a folder named "__MACOSX".
If you prefer to use Stuffit Expander most of the time, just do a Get Info in the Finder on the specific .zip file to unpack, and under the "Open with:" section, select "BOMArchiveHelper". Then double-click it as normal. If you want to switch all .zip files to use BOMArchiveHelper (which is recommended, since this can unpack Stuffit's and other .zip files fine), then also click the "Change All..." button in Get Info.
BOMArchiveHelper not listed? Choose Other..., then navigate to <Your Hard Disk>/System/Library/CoreServices/BOMArchiveHelper. Highlight it, then click "Add".
To prevent Stuffit from reclaiming the .zip extension if it's run on another type, launch Stuffit Expander, go to Preferences, Internet, and uncheck the "Zip" checkbox in the list. This is in addition to the Change All step above.
* the BOM in BOMArchiveHelper stands for "Bill Of Materials", as it's used by the Installer and Software Update features. It would be nicer if Apple could give it a less nerdy name though.
© 2006-2008 Graham Cox