A .sit file is simply a compressed, or "zipped" file. If you are a Mac user the Mac OS can decompress it for you, just double-click it. Otherwise you will need to download the free StuffIt Expander.
Stuffit Expander for Mac: http://my.smithmicro.com/mac/stuffit/expander.html
Stuffit Expander for Windows: http://my.smithmicro.com/win/stuffit/expander.html