Hi Perfo,
I just downloaded it from the Mach site and tried it and it works fine. I thought that maybe the file was damaged on the Artsoft site, but it appears fine. Try downloading it again. Yes, it goes in the C:\Mach3\Plugins folder (or whatever drive and path you are using).
What version are you using?
One thing that is different is that, normally, Artsoft will convert dll files into m3p files that you download and click and they will self install into the plugins folder. This time, they just put the DLL there directly. You said that you received a Mach4 dll faulty message. Do you mean Mach 3?
Anyway, try downloading it again and let me know.
Lee