The vocabulary is available at http://purl.org/net/dssn/.
You can use it in Turtle with
@prefix dssn: <http://purl.org/net/dssn/> .
or RDF-XML with
<?xml version="1.0" encoding="utf-8"?>
<rdf:RDF xmlns:dssn="http://purl.org/net/dssn/" … >
…
</rdf:RDF>