Hi Curt,
The NI-DAQmx C Reference Help (located under at Start >> Programs >> National Instruments >> NI-DAQ) shows that the DAQmxCreateAIVoltageChan function has a customScaleName parameter. Most Analog Input configuration functions also have this parameter. This is how you access scales built in MAX. Please see the NI-DAQmx C Reference Help for more information. Specifically, look under NI-DAQmx C Functions >> Channel Configuration/Creation >> Create Analog Input Channels.
Let me know if you have any further questions.
Regards,
Sean C.