One of those is Battery +, you don't want to be soldering that to ground.
About the best option there is to just 'hard wire' the battery pack to the board. You'll never get that connector to stay on there past the first person that tries to unplug the thing.
The thick trace there in the middle of the board is the Batt + line, which Daywalk3r marked on your pic, then use any Ground for the Batt - connection. Just clean the board up good and make sure that Batt + line isn't shorting to ground before you solder on the battery wires, or you'll find out it was the hard way.