the wires that im speaking of are the one for the ground cable and the one that powers the mig gun. these will usually be connected to terminals inside the door of the machine labeled positive and negative. if you are welding with shielding gas you'd want the electrode wire (the one to the gun) connected to the positive terminal and the ground wire connected to the negative terminal. since you are welding with flux core wire you'll want to make sure that the gun is connected to the negative terminal and the ground connected to the positive.
in my experience, some flux core wire is garbage. I actually made the mistake of buying it at harbor freight once and literally returned most of a 10lb roll because it was unuseable. Ive had good luck with hobart and lincoln wire but i rarely weld with flux core anymore unless im welding something outside.