How do I open a .sit file on Mac OS X?

I downloaded a program, and to run it without the CD I need to somehow use a file that came with the download, but I have no idea how to open the file and apply it to the program. It’s a .app.sit file and when I double click on it it opens in text edit and is just a bunch of gobbledigook, funny characters and stuff.

Anyone know what I’m talking about or what I need to do? Sorry for the vagueness, I’m really not very tech savvy.