Usually SIT files are associated with the Mac operating system. SIT is Stuffits' equivalent of a ZIP file. Open Type and Truetype are cross platform fonts, in other words you can use them on either a Mac or PC. Postscript fonts are not, they are platform specific
A postscript file is usually made up of 3 parts, only 2 of which are necessary. PFB, PFM, and AFM. The AFM file is not required. So if the font is in fact a Mac font then that is the reason you cannot use it on your PC. There are commercial and freeware programs out there that allow you to convert PC postcript fonts to Mac postscript and vice versa.
Do a google search for a full detailed explanation and to maybe find the proper software to make it work for you.