That's silly. You do not interface an instrument with an RS-232 port to DAQmx. DAQmx is a driver for data acquisition boards from NI. You connect the instrument to the pc's com port with the appropriate cable and use VISA Reads and Writes to communicate to the instrument. In LabVEW, you have a couple of examples for serial communication. I would also recomend you use a terminal emulator program such as Hyperterminal to test the connection before trying to write any code.