Unzipping is not part of Dreamweaver. It's done by your operating system or by a third-party utility such as WinZip or Stuffit.
On Mac OS X, ZIP files are automatically unzipped as soon as they're downloaded. You should see a folder with the same name in the Downloads folder.
Instructions for Windows 8 are here: http://windows.microsoft.com/en-gb/windows-8/zip-unzip-files.