Hi Jim,
This depends on what type of PC you are using. If you are using a PXI chassis as a real time environment and targetting it from your host PC, then there should be no problem. Check that you have all of the drivers installed correctly.
If you are using a normal PC as a real time system, this is unfortunetly not supported by National Instruments and we have been unable to get it to work.
Please follow this link to see which boards are supported in LabVIEW Real Time for different systems.
http://www.ni.com/support/labview/real-time/drivers.htm>
Hope this helps,
Best regards,
Peter H
Applications Engineer
National Instruments UK