You need the Mac Stuffit Expander (WinZip equivalent) to expand a .SIT file on your Windows pc. Goto:
> Z – Tools & Utilities
> = Freeware
> Aladdin Expander Stuffit
and download the free Expander.
Then simply install the Expander application, expand the file .SIT, add the extension .MYO to the data filename and open this in the compatible Windows version of MYOB.