Hello Adam,
PXI-6704 is not supported in DAQmx. Please refer to the following link.
http://www.ni.com/support/daq/versions_pci_pxi.htm
However, if you are interested in traditional, please go to NI Example finder, and search for PID, and the very first example - One Channel PID Control.vi - will give you a flavor of how to program a DAQ board in RT.
Regards,
Arun V
National Instruments