It's only a 3 pin serial connection so this would imply that you only need ground, Rx and Tx. The pinout on the db9 connector is:
pin 2 - Rx
pin 3 - Tx
pin 5 - Gnd
I'd imagine the black wire is ground with Tx and Rx being on the 2 remaining wires. It doesn't usually matter if you get Tx/Rx around the wrong way but it won't work properly. If this happens just reverse the Tx/Rx and it should work okay.
Of course you do this at your own risk, but if someone could give the exact pinout it'd be much less risky