You may want to try seeing if port 9100 is open on the router as it may simply implement a config-less RAW (read after write) printing gateway. There are plenty of guides to setting up RAW TCP/IP printing on the web.
However, a quick google did point me to one thing... It's been mentioned that the 655 is the model which supports printer sharing (not the 615), that DLink have simply used the same case to cut costs and that the USB port on the 615 is not actually connected to anything...