Drag and drop that file onto Stuffit Expander, and it should decode automatically.
If you have any problems unstuffing that file, be sure you have the latest version of Stuffit from Smith Micro.
OS X 10.4 or later does not come with an installation of Stuffit Expander. You will have to go to the above address and download the free version of Stuffit Expander. You can also use the open source alternative, The Unarchiver.
The files on our site that are .sit files have to be in that format as they run on both OS X and OS 9. OS 9 will never recognize .dmg files, the standard format for our OS X only products.