A .sit file downloaded on a Wintel box, transferred to a ISO CD, and then upacked on a Mac should work fine. Under no circumstances should you try to unpack on the Windows machine, as that will screw up the resource fork big time. A .sit has no resource fork, so it is safe to dl regardless of platform.

Usually, just dragging the .sit file to Expander (I've never encountered the problem that Joakim discusses) works fine. Nor have I ever experienced any complications with the pay versus free version of StuffIt, since they use the same components. You can also add the .sit suffix in File Exchange if it is not there already.

If you do, however, have the pay version, you might send an email to Aladdin explaining your problem if no other solution works. And if I hear "zap the pram" one more time...