I also was looking for an address in Beijing, but I don't think China Southern has an official office in Beijing. I found out, however, that there is a China Southern Business Pearl hotel (or pearl business, not sure) with a customer service desk. They can help you with your ticket stuff as well. Good luck!

For your reference, the office address for China Southern Airlines in Beijing: China Southern Airlines Mansion, No. 2 East Third Ring South Road, Beijing.