Guys, solved the problem!
I was supposing that the UART was dedicated to communication with the PC. However, in Leonardo version, the Atmel's chip is USB-built in, so, the UART is not dedicated to PC, and to gain access to data on the RX and TX pins (digital 0 and 1 pins) you have to instanciate the UART as Serial1.
So, you inicialize Serial1.begin() and read/write using Seria1.read()...
Because of this USB-built in feature, you can even read the UART and print in the serial monitor, using the example below:
Serial.begin(57600);
Serial1.begin(9600);
/*your code*/
a = Serial1.read();
Serial.println(a);
/*your code*/