1. Download the StuffIt (.sit) files.
2. Expand .sit files, which will then place an empty folder on your desktop with the icon applied to the folder.
3. Click to highlight that folder on desktop, then press cmd-I.
4. Click to highlight icon in info panel, then press cmd-C to copy.
5. Find desired folder or HD you wish to apply the icon to and repeat step 3 to open info panel.
6. Click on existing icon and press cmd-V to paste new icon over the top.