If you are talking about the via6421 pci sata card, it would go in an empty pci slot in your motherboard.
the exact location depends on your specific motherboard, but its the PCI slots. typically these are the slots that are right next to the back of the machine and can be connected to through the back of the computer. Like the cable input for example
i believe you are supposed to use the last one. not sure which end though, guess you'll find out through trial and error, if the pci isnt free, make it free.