Thanks for sharing your application. Let me first verify that I understand correctly what you want accomplish. First, you want to generate a signal with the NI Signal Generator, what specific device are you using? Then you will be measuring the reflected signals from your DUT with the 6259. You don’t give much information about the frequencies, sample rates and other parameters involved; so I understand that you already took into account the frequency at which you want to generate and verified that the Signal Generator can accomplish your requirements, also that the USB-6259 is capable of acquiring the signal that will be getting back from the DUT.
At this point, I would recommend looking at the examples (Help>>Find Examples) shipped with DAQmx (Hardware Input and Output>>DAQmx) and NI-FGEN (Hardware Input and Output>>Modular Instruments>>NI-FGEN). There you’ll find examples about signal acquisition that might be helpful for the 6259 part of your application; also you will find examples for generation using the Signal Generator and also for frequency sweep. unr, I hope this give you a good place to start, please post back If my assumptions are mistaken or if you have more questions.