Figure out which of the two wires is connected to the center connector on the DC power plug. Insert this through the PC board hole labeled V+. To make sure, put your multimeter into continuity mode, put one probe into the center of the DC power plug, and touch each of the two leads at the other end. The one that beeps goes to V+.

The other wire from the DC power cord goes through the PC board hole for the unlabeled negative power terminal. You might need a solder sucker to open up the PC board holes before you can insert the wires.

If you're like me, you want to use your multimeter again, to confirm that the center (positive) part of the DC power plug connects to the hole on the PC board labeled V+. Then solder the two wires securely to the PC board.