Greetings delpitik,
Thanks for contacting National Instruments. The appropriate pin for your connection would be AI GND (pin 76). This information can be found by right clicking on your device in MAX and selecting Device Pinouts. Also, when you create a new DAQmx task in MAX, you can also specify the type of analog trigger. (i.e. edge, window, etc)
Thanks and good luck!
Jordan Randall
National Instruments Italy