What file are you talking about, specifically? I have MiamiDX and use it regularly, just did a quick search on my SYS: partition and didn't find anything named "Miami-Mui-Gui".
If your install of it is mucked up you may just want to re-run the installer, in my experience Miami is probably the easiest of the Amiga TCP stacks to install & configure.
(I know, I'm opening myself all up to flames by saying that, lol) :p