Hey Blau,
The option to do analog triggering is going to be dependent on whether or not you are using a device that supports it. You might want to locate your device on http://www.ni.com/products, and see whether or not this is a supported option for you.
The following tutorial talks more about analog triggering with E Series devices: http://zone.ni.com/devzone/conceptd.nsf/webmain/2AF60E522378449286256815005806D3?opendocument. You also might want to search the NI Developer Zone for more tutorials and even example code on using an analog trigger.
Hope this helps.
Jared A.
National Instruments
