The red wire coming off the battery is positive (+). The black is negative (-). You want the outer ring of the DC plug to be positive (+), the inner ring to be negative (-).
Two ways to get the polarity correct:
[1] Check the voltage polarity of the DC plug while hooked up to the battery
Wire up the DC plug temporarily to the battery - no solder or crimps, just twist the wires together. Hook the battery up. Place the red tip of your voltmeter on the outer ring of the DC plug. Place the black tip of your voltmeter on the inner ring of the DC plug. If the voltmeter reading is positive, then you have the DC plug wired up for outer ring (+), inner ring (-), which is correct. If the voltmeter reads negative, then the opposite is true: the DC plug is wired for outer ring (-), inner ring (+) - in this case, switch the wires so the outer ring is (+), inner ring (-).
[2] Check the continuity of the wires leading to your DC plug
Set your multimeter to check continuity. Place one multimeter lead on the outer ring of the DC plug. Place the other on either of the wires leading to the DC plug. Whichever wire provides continuity is the wire you want (+). The other, obviously, (-).