The pcmcia card is used to connect via DH+, the pcmcia cards are quite expensive. If your processor has an ethernet port you can connect via ethernet, no additional hardware required, but you will need to know the IP address of the PLC. I think all the PLC-5's that have ethernet also have a serial port. So the easiest way may be via serial connection, but it will be somewhat slower. You can also use the serial connection to determine the IP address.
Do you have a backup copy of the PLC program, the IP address can be gotten from there as well.
If you've never done any of this, it may be best to have someone from the outside come in and go over it the first time. It will save a lot of headaches. Ask lots of questions, and take good notes.