http://en.wikipedia.org/wiki/D'Addario"The D'Addario (phonetically pronounced /dəˈdɛɹio/ in American English or /dadˈdaɾio/ in Italian) family of string-makers originated in the small Italian town of Salle in the province of Pescara."
So dad-DA-ree-oh (Italian) or duh-DER-ee-oh (American)