You seem to be guessing at the command as well. What does the manual say? With a serial instrument, I always recomend that you first try communicating with Hyperterminal. If you can't get that to work, you might have the wrong port settings or the wrong type of cable. You are also more likely to get help from the vendor if you mention Hyperterminal. A lot do not understand (or even know about) LabVIEW.
Also, is it the EG&G instrument? If it is, download the driver I mentioned.