For me, when I tried to convert MusicXml3.0.xsd , xsd.exe each time. So what I did was
xsd.exe musicFile.xml
which generated the schema file, and
xsd.exe musicFile.xsd / classes
which created the required c# classes. The only thing I had to add was that the generated xml should be read by programs like Sibelius 7 should add
<!DOCTYPE score-partwise PUBLIC "-//Recordare//DTD MusicXML 2.0 Partwise//EN" "http://www.musicxml.org/dtds/partwise.dtd">
And presto!
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