Referencing a field in a subdocument appears to be broken in version 3.0.4 of mongoexport.
In previous versions -f subdoc.0.fieldname would export the values in the first element of subdoc. This is no longer working since I updated to 3.0.4 from 2.6.4. The following is from the docs and it states that the field can be within a subdocument.

--fields <field1 [,field2]>, -f <field1 [,field2]>
Specifies a field or fields to include in the export. Use a comma separated list of fields to specify multiple fields.

For csv output formats, mongoexport includes only the specified field(s), and the specified field(s) can be a field within a sub-document.