That's odd. Normally StuffIt Expander will open it. But if not, then drag & drop it onto Expander. It sounds like you don't have a default set up so perhaps on this occasion, select the SIT file, Control click it & select open with & choose always with. (b) * Else, select the SIT file, then apple & i keys to get info. Then choose open with. And select StuffIt Expander & select Change all to open with.

If you don't have stuffit expander then get it from here: http://www.stuffit.com/mac/expander/ (the expander is free!)

BTW sometimes StuffIt expander doesn't work when you double-click the sit file. This seems to happen if your comp has been on for days & been running a large range of heavy duty programs. In that case drag & drop the file works. Else reboot.