hey, thx for your answer! What do you mean by "have you looked at which pins are common on the breadboard?"
Anyways, I think I missunderstood the breadboard: it seems only the outermost two columns are connected vertically and the ones in the middle are connected horizontally.
So I corrected my wiring and it is still wrong. I am trying to implement the sketch that you can see in the attachements as well. This is a sketch for the Arduino Uno and I am trying to implement it on the Mega2560, but it shouldn't be of any difference now, should it?