By the way, this FAQ is quite outdate, eeryone should be using Stuffit 9. Where is Belthazar when we need him to copy-paste his usual answer? At any rate, here is it:
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.
This function is now built into DA's MissionComputer app, too; there's a handy "Save as .rez" command right there in the file menu.