The reason that the DAQmx Assistant does not recognize the AT-MIO-16XE-50 is because that board is not supported in DAQmx. All of the AT boards will only be supported with Traditional NI-DAQ. I would recommend looking at some of the Traditional NI-DAQ analog output examples in order to get started.
Also, if you would like to see a list of all the boards supported under DAQmx, check out the NI-DAQ readme file which comes with each version of the driver. If you already have it installed, it should be here: C:\Program Files\National Instruments\NI-DAQ\readme.html.
Hope this clears this up.
Jack