I got the airtunes to work, but when I plugged my printer USB cord into the USB slot on the Base, it would not print wirelessly. When I plug it into my MBP it works.
Did you try configuring the printer in the Airport Admin Utility or try finding the printer in the Printer System Preferences?
EDIT: I just tried this and you have to add a new printer in your Printer System Preferences. It will show up as connected to your Airport Express. Works like a charm!