Check the Epson support site and download any printer driver updates that might be available for your printer.
Not all printers will work when they are connected to the AirPort Extreme, since the manufacturer(s) may not provide network USB drivers for all of their printers.
When you click the + button to install the printer at the AirPort Extreme, the name of the printer should appear immediately. If it does not, that indicates that the network USB drivers have not been downloaded, or they are not available from the manfacturer.
This Apple Support document may provide more information about troubleshooting:
http://support.apple.com/kb/TS1253
Also, keep in mind that only the print function is supported at the USB port of the AirPort Extreme. If you have an All-in-One type of device, you will need to connect it directly to your computer for scan, copy, maintenance functions, etc.