failed to connect to xbee

I am trying to follow this tutorial :http://wiki.ros.org/rosserial_xbee/Tutorials/Example%20Network However, when i run the command rosrun rosserial_xbee setup_xbee.py -C /dev/ttyUSB0 0 for configuring my xbee, i get an error saying "Failed to connect to the xbee." i am using the sparkfun xbee explorer. WHat am i doing wrong? The xbee is conneced to /dev/ttyUSB0