That's caused by the version of lxml being too old in the calibre OS X build, it will be updated when the next calibre release is made. In the meantime if you want to workaround it, you can replace the line
Code:
descendants(doc, 'w:p', 'w:tbl')
with
XPath('descendant::w:p|descendant::w:tbl')(doc)
there may be other places where you will need to make the change as well.