Usually the archives on MAC could be extracted with the default extracting utility. Just double click on the downloaded archive package and it would be extracted into the same directory.
To extracted the files with Stuffit Expander software open the Stuffit Expander, and holding the mouse drag the archive package to the Stuffit window.
The extracting will begin.
When the extracting is complete you can find the extracted files in the same directory with the downloaded archive package.