Rolf,
Most likely, you are unable to access the Coupling property because none of the devices in your system support it. If you right-click on the DAQmx Channel Property Node and choose the Select Filter option, by default it is set to "Show Attributes For Configured Devices". If you select "Show All Attributes", you will find the Coupling property located under Properties >> Analog Input >> General Properties >> Input Configuration.
Good luck with your application.
Spencer S.