I've just gotten a fully assembled rotoshield and connected it to my Arduino Mega 2560 Board. I've also connected 4 small DC motors and a ca. 7 V battery pack to the external power supply.
When I turn on the power to the main board (mega 2560) the blue LED lights up. But nothing happens when I upload a simple sketch like this:
#include <Wire.h>
#include <snootor.h>
SnootorDC M1;
SnootorDC M2;
SnootorDC M3;
SnootorDC M4;
void setup(){
Serial.begin(9600);
Serial.println("Motor test!");
Wire.begin();
M1.init(1);
M2.init(2);
M3.init(3);
M4.init(4);
M1.setSpeed(200);
M2.setSpeed(200);
M3.setSpeed(200);
M4.setSpeed(200);
M1.run(FORWARD);
M2.run(FORWARD);
M3.run(FORWARD);
M4.run(FORWARD);
}
void loop(){
Serial.println("forward...");
delay(2000);
}
I do see the "println" stuff when running the serial monitor.
REMARK: The blue LED does not light up when I turn on the external power supply (even though the polarity is right), but it does turn on when I turn on the power supply for the main board!
Furthermore, when I check the voltage differences it looks like there is a missing link somewhere. It's like the circuit is broken, but I don't know enough electronics to find out where

Do you think you'll be able to help?
Thanks,
Erik