The null modem cable should cost the same as a regular serial
cable so unless you already have a regular cable, buy a null
modem one. If you do already have one, on one of the connectors
swap pins 2 and 3. This switches the transmit and receive wires
making a null modem cable. If this does not work, it is because
the serial port is trying to use some of the other handshaking
pins. You can look in a store like Radio Shack for how to
fake out them. You should be able to get a 3 wire cable to
work with pin 2 connected to 3 and vice versa and finally a
ground wire ( from memory ground is pin 7 so connect pin 7
to pin 7).
Good luck