edit: nigel beat me to it...
a null modem cable is for communicating between one PC and another via the serial port. The TX and RX pins are crossed over, so the TX on one end goes to the RX on the other end, and vice versa. Thus, it's not going to work for you. However, you can rather easily cut open the cable, find the TX and RX wires, cut them, and resolder with the connections swapped. I've done this many times for crossover network cables. It might be a bit of a pain to find the wires, because it's not as easy to find the color code as with network cables, but it's doable... though if you can easily buy the right cable instead that's probably preferable.