With the DAQ Assistant and a digital output task, the data input expects an array of Booleans. This is default behavior and not really all that useful for controlling just a single line. What you should do after configuring the assistant, is go to the front panel and create a Boolean control. Any type of Boolean will do. If you do not know how to create a Boolean control or do not what the front apnel is, you really, really need to spend some time with the LabVIEW basics class.
Anyway, after you have the Boolean control, go to the block diagram and from the Array palette, place a Build Array function on the diagram. Wire from the Boolean to the Build Array input and from the Build Array output, wire to the data input of the DAQ Assistant. Go to the front panel and set the Boolean control to what you want. Run the VI. If you have a scope or DMM connected to the DAQ card output, you should see the output change when you change the front panel control and run the VI.
You might also want to mention the type of DAQ board you are using if you have any problems.