There is a LabVIEW shipping example for DAQmx called "PWM Counter Output". This is the easiest way to generate pulses and update the frequency and duty cycle. This particular example uses an analog reading to set the new frequency and duty cycle, however this portion of the code can be removed to update these values using front panel controls.
I notice however that you are working with LabVIEW 6.1 which does not support DAQmx, only Traditional DAQ. The LabVIEW shipping examples are not as extensive for Traditional DAQ. You can search ni.com for examples for Traditional DAQ. Here is a link to one such example to get you started if a newer version of LabVIEW is not an option for you:
NI-TIO and DAQ-STC:Pulse Width Modulator.
Hope this helps,
Jennifer O.
Applications Engineer
National Instruments