Good advice from Belthazar, on decompressing and converting Mac format plugins on a Windows machine:

This is a problem you'll only encounter on a PC. On Macs, they have two forks, a resource fork and a data fork. For plugins, all of the plugin resources are stored in the resource fork, logically enough. On the PC, we only have a data fork, meaning that when plugins are incorrectly expanded, the resource fork is destroyed, leaving a file of zero-byte size.

To expand a .sit or .sitx archive, download Stuffit 9 from http://www.stuffit.com and install. Once Stuffit 9 has been installed, just right-click on the file and select Stuffit Archive > Expand Mac Archive > MacBinary Fork Encoding. This should give you one or more .bin files, and (if the plugin maker is any good) a readme of some kind. Drag and drop these .bin files on the convertor (it's named "Convert Plug-ins.exe") - you'll know if it works because the DOS window that pops up scrolls some text across it quite fast, and a new .rez file will appear which has the same name as the .bin file you put in. You may also get a file called resource.map, which should be deleted.

If the archive is a .hqx archive, you'll need to select Stuffit Archive > Expand here when you right click, in order to get a .sit archive, then you can perform the expansion as above.

Note that the .bin files will still claim to be Stuffit archives - this is because there are Stuffit archives with a .bin extension, and there are even some of these on the Addon pages (just extract as you would a .sit archive; it gives errors, but it still works). However, the .bin files which you get after expanding the .sit archive are not archives, but are MacBinary files (hence the .bin).

Rule of thumb: if it's a .bin when you download it, it's probably an archive. If it's a .bin after you've done the expansion, it's probably not an archive. Another way of telling is that if you expand a .bin file and get a zero-size file (regardless of how you expanded it) then it wasn't an archive.

If you wind up with any .DS_Store or _Icon files after expansion, delete them, as they're Mac-only thingies.