Done!, what hapened this time is that this display uses negative voltage for contrast adjust, so you have to slightly modify the tutorial on lcd screens, you haver to attach -5 v to one side of the potentiometer, gnd to the other side, and the middle pin goes to the contrast adjust (pin 3 of the lcd screen) but anyway, thanks for your fast answer!
Now I will have to find a way to supply -5 volts to the potentiometer, because right now Im doing it using an atx power supply haha