It looks like MXD files only store information about the project (eg
what layers you have, what symbols to use etc) not the actual spatial
data so you wouldn't be able to convert them without having access to
all the data the project was loading. If you have ArcMap then you
should be able to use it's kml export tools to produce something
google earth can read; or at least export the data to shape files and
then use any of the tools out there for converting those to kml.
See here for answers to a similar question about importing into
AutoCAD:
http://discussion.autodesk.com/thread.jspa?threadID=434023
Cheers,
Chris