The best way to troubleshoot a hardware problem such as this, is with the Measurement & Automation Explorer (MAX). Once MAX is open, go to "Devices and Interfaces". Right click on your DAQ card (PCI-6025E) and select "Test Panel". Now, select the Analog Output tab, set an output voltage and then press "Update Channel". Also, consult your User Manual to make sure that you are connected to the correct pins i.e. DAQ0OUT and AOGND.
Regards,
Brian Anderson
Applications Engineer
National Instruments