We don't use the word execute on mac.

A SIT file is the mac equivalent of a ZIP file.

Normally you'll get StuffIt Expander with your mac so you can uncompress or expand the sit file. Double-clicking the sit file or drag & drop the sit file onto stuffit expander. Normally there will be a stuffit expander alias (mac name for shortcut) sitting on your desktop. If you don't have stuffit you can get it from here: http://www.apple.com/downloads/macos...texpander.html
It is free unless you want to stuff (compress) files which is a shareware portion of the program.