"Message not understood" means an object is being sent a message that isn't defined. If it's "Message not understood: #line", that's because your file is missing its first line, which should be something like this:
!MuxReader class methodsFor: 'scanning'!
There's also a whole bunch of other stuff that this method refers to that isn't in that file - the MuxReader class definition, a MuxReader class method called connect, instance methods called streamIn, streamOut, process:, and scanLine:, plus another class UIGoogleMap with instance methods open and changeMousePositionFromName:.
It looks like VisualWorks Smalltalk code, or at least I can't see anything there that would point to another dialect. Since it's in chunk format (delimited by !) rather than XML, it looks old school, but the reference to UIGoogleMap means it can't be all that old, so maybe just an old-timer like me who prefers the simplicity of chunk format over XML :). I tried a few web searches for the non-base methods, but couldn't find anything, so hopefully you have access to the rest of the code already - there's not much more we can say without it.
All the best,
Steve