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Have you initialized the object Serial?

Somewhere inside the setup() function should be a line like:

Serial.begin(9600);

Hope this helps

~~Have~~ I have never used rosserial_arduino, but I used arduinos before, have you initialized the object Serial?

Somewhere inside the setup() function should be a line like:

Serial.begin(9600);

Hope this helps

UPDATE

Forget about what I said before. I don't think using Serial.print is a good idea as it might interfere with ROS's communication. Have you tried blinking the LED instead?

digitalWrite(13, HIGH-digitalRead(13));   // blink the led

You should see the LED blinking quite fast