Have you successfully communicated with this board before?
How is the board powered? (USB or via DC power jack)
I successfully communicated with this board during 1 week, and other boards have no pb.
I'm using mostly USB, but tried with DC power too.
I tried without my ethernet shield, without protoboard, really alone, and no RX/TX led, no detection in device manager.
In hurry, I bought an other one in adafruit shop :)
But I'm very disapointed for the other one.