The most straightforward thing to do is to get a serial port for your computer. You can buy a PCMCIA-type or PCI-type serial expansion card, or a USB/serial converter.
I recommend the Keyspan HS-19 USB/RS232 converter. They are inexpensive and popular and have worked very well for me with RSLinx and other programs. They even support the "autoconfigure" function in the DF1 Full Duplex driver, which makes configuration nearly foolproof.