Any zip archives distributed by Synthetik Software for the mac are generated using the built in mac OSX zip compression. If you use Safari to download a zip archive, it should download it to your download folder, and then decompress it automatically. If a zip archive is not automatically decompressed for some reason, you can do so by double clicking on the zip archive. This will run the built in OSX Archive Utility program to decompress the zip archive.
Zip archives that are attached to emails are stored on your computer in the /Users/yourname/Library/Mail Downloads folder.
Do not use third party zip decompression applications like Stuffit to decompress zip archives. They do not work reliably under osx. If you have your mac configured for some reason to use third party zip compression programs, you can override that behavior by selecting a zip archive, control clicking it, and then choosing the Open With : Archive Utility context menu command.
If you try to decompress a zip archive on mac and it doesn't work,the problem is that you are suing a third party application like Stuffit to do the decompression. Using Archive Utility resolves the problem for customers.