Hi WU undergrad-
NI-DAQmx does not support versions of LabVIEW earlier than LabVIEW 7.1. Write to Digital Port is a Traditional NI-DAQ function and is not supported by NI-DAQmx. In order to use your USB-6501 with LabVIEW you will need a newer version. Once you upgrade, you can find examples in the NI Example Finder under Help>>Find Examples in LabVIEW.
Hopefully this helps-
Tom W
National Instruments