Hi,
What do you mean by initialize? Do you mean setting each of the lines low or high? What language are you using to program the PCI-6703? If you are using LabVIEW you can go to Help>>Find Examples to find digital output examples. There is a section in the bottom right corner which lists supported devices. There is a DAQmx Write Digital Port example that works with the PCI-6703 that you may want to look at. This example will update the digital port to correspond with the value you enter in the "Data to Write" control.
If you are working with CVI, C or VB, installing DAQmx should have also installed examples for you to reference. These examples are located in the NI-DAQ folder (i.e C:\Program Files\National Instruments\NI-DAQ\Examples).
Hope this helps,
Jennifer O.