As stated above, I have a 2.1mm barrel jack designed for a breadboard. The jack has 3 pins which insert into the main part of the breadboard (Not power rails). The datasheet does not state where my wires need to go to connect positive or earth/negative. I assume it requires connections because it does not fit into the power rails.
The datasheet shows pin 1 as the centre pin, pin 2 as the barrrel and pin 3 connected to the barrel when the plug is not inserted. Pin1 is on the centreline furthest from the plug, pin 2 on the centreline nearest the plug, and pin 3 off to the side. Which is positive or negative depends on how the plug is wired.