Solution
By default, LabVIEW does not search for cards in you Real-Time PXI system.
To select a card or channel of your PXI system, you have tell LabVIEW about your Real-Time system.
To do that, you start with using an Real-Time LabVIEW project from the templates,
or you
add a RT target to your LabVIEW project.
After that, you have to add the VIs, which will control the PXI cards, under the RT Target in the LabVIEW Project.
Now you should see the cards in the drop down menu.