Which GND pin do I use on a DAQ?
DGND and AGND are both ok grounds to use. Depending on MyDaq breakout box configuration they are  most likely coupled together internally and there is no harm tying them together under most set ups. The difference is there to avoid mixing signals with different noise sensitivity by splitting up their current return paths. Digital signals are noisy and the voltage they generate across small impedance in the ground return path can couple into sensitive analog signals sharing the same ground. 

In more complex set ups, for instance where you are using your Daq to measure voltage levels as well as control digital logic, you would need to think about your ground wiring to make sure everything operates with minimal noise and error. 

Otherwise please review your manual since they will have application suggestions and typical wiring diagrams.