I'm not a Mac expert, but I believe that the 9 pin serial Mac connector that is on some Macs can be easily converted to a DB-9 with an off-the-shelf cable. LabVIEW also contains NI-VISA and traditional serial vi's that allow communication with the native serial port. If your Mac does not have native serial ports, then you will have to purchase an off-the-shelf USB to RS232 convertor for the Mac. The HW should come with a driver that will map the serial port to a local port which can then be accessed by LabVIEW/VISA. As far as PCI cards go, NI-Serial (PCI) is not supported under Mac, and I'm not sure what other PCI options might be out there.
"Would you like to take a survey?"
What type of interface to RS-232 would be most useful for your application:
PCI, USB, or Ethernet?