SerialUSB on Arduino Due

I'm starting fiddling with a Due but I can't seem to be able to communicate over USB with it. Here is my code:

int bauds = 9600;

void setup() {
    SerialUSB.begin(bauds);
}

void loop() {
    if (SerialUSB.available()) 
        serialUSBEvent();
}

void serialUSBEvent() {
    String str = SerialUSB.readString();
    SerialUSB.write(str.c_str());
}

As you can see, I try with the bare minimum but my output consoles (2 COM port for the 2 usb connections, just in case I messed up which is which) stay blanks, whatever I send them...