Alternating current (ac) doesn't have polarity so there is no plus or minus. But AC does follow a path, so you might as well hook up the appliance the right way. Buy a power cord and it will have a black, a white and a green wire inside. Hook the black, white and green wires in the appliance to the same colors in the power cord and you will be fine. If the wires inside the appliance are not black, white and green, look at the schematic for the appliance are determine which wires are hot, neutral and ground. Hook these wires to the power cord with black hot, white neutral and green ground.

The appliance will work with the black and white wires reversed, but it's much better to keep all your electrical stuff connected in the same manner - and it's just as easy to hook it up correctly as incorrectly.