Hello Kande,
The DAQ Assistant may only be used with the MIO series of data acquisition devices. This includes all E Series and M Series devices support by DAQmx.
If you are looking for help with getting started with DAQmx and the PXI-6704, I suggest opening the Example Finder found at LabVIEW Help >> Find Examples and browsing to Hardware Input Output > DAQmx to find an appropriate analog output program for that device. The other method that you could use is instead of trying to use the DAQ Assistant in LabVIEW, create a DAQmx Task in MAX to be used later on in your LabVIEW code.
Jared A