I assume the converter came with an AC line cord and plug. If so that is a direct plug into an AC socket.
On the output or DC side you should have at least 2 large diameter wires and maybe a bare copper wire tied to the chassis. If there is a bare copper wire you should find a single screw lug most likely on the bottom of the converter frame and also most likely at the opposite of the converter from the 2 battery terminals. The bare wire goes in that lug and grounds the converter.
The 2 large wires going to the batter are critical as to which one goes to which terminal.
If you have a volt meter, and with the converter plugged in, place the common probe, black, on the copper wire and touch each of the output terminals with the other probe, red. One of those terminals will show voltage, about 13 volts. Note that terminal. With a battery installed now touch each of the large wires coming from the battery. One will show voltage, about 12 volts. Now connect the terminal/wire that showed voltage together and then connect the terminal/wire that did not show voltage together. Now check the voltage on the battery. I should be as about as the converter showed when you tested it. It may be slightly low but will come upi with time. If the battery voltage is below either of the voltages you measured before connecting up the wires are backwards on the converter.
WBCCI 12156 AIR 3144 WACHUNG TAC NJ6
2004 Excursion 4x4
1991 34 ft. Excella +220,000 miles, new laminated flooring, new upholstery, new 3200 lbs axles