I solved it using an USB Shield for Xbee module directly attached at my PC. I wasn't able to rewrite the firmware of the module on my arduino because the arduino proto shield has only four communication pin (5V, GND, TX, RX).It is perfect for communications but not for writing the firmware, you need all twenty pins communication. For the technique I rewrites the module entering in bootloader mode. Hoping it could help for others with the same problem!!