[Edit: Posted at the same time as spiralx's message; sorry for the repetition.]
JimDot, if you follow the link at the top of this thread -- smartftp.com/client/addons/sourcepreview -- then you get an installer that you just have run, after which it should just work.
You don't need Scintilla.dll; you don't need to copy SourcePreviewHnadler.dll anywhere (the installer copies it to where it needs to be). AFAIK you don't need the msvc*71.dlls either (or if you do the installer will take care of them).
None of those DLLs should be placed in the Opus plugins folder; if you've still got copies there that you made yourself you should delete them in case they cause problems.
You said, "as others have posted, I'm unable to get this viewer to show up in the list of viewers," but I don't see anyone else posting about problems with this viewer, only about the other viewer in the other thread. That other viewer needs Scintilla.dll but this one doesn't. That's why I assume you're confusing the two viewers/threads. (Edit: Having re-read this thread, there was one set of messages about problems with this viewer, but I that was just confusion about it not being an editor (like the other one), just a viewer.)
(There is some mention of Scintilla in this thread but ignore that; it's just people talking about other similar components.)