Hi there,
No worries, the Mac version is super-simple

A .sit file is a StuffIt archive. You should just be able to double click it and it will unstuff the archive and create a folder with the files. If double-clicking the .sit file doesn't work, you will need StuffIt Expander (it's free).
After you have unstuffed the file there will be a folder with a ReadMe file and the app.
If you have any additional problems or have questions after that, feel free to post

S