The 1784-PCMK/B card is for DH+ and DH485 communication. Your ML1500 has 2 serial ports, you do not need the 1784-PCMK card to communicate with your ML1500. Your processor has 2 serial ports one 8 pin one 9 pin. Simply use a null-modem cable from your computer to the processor. See PDF below for more on the cables.
The cable you have is for DH+ networks, it's a good thing it did not fit you could have damaged the serial port on your ML1500.