I am not sure what driver for the instrument you are using, but there is a LabVIEW GPIB instrument driver for your instrument (written by the instrument manufacturer; see details at the site below) located at: http://zone.ni.com/idnet97.nsf/9b2b33e1993d877786256436006ec498/966189a0e0d39bed86256d8a0068b2c8?OpenDocument
If you have GPIB hardware you may consider this. Otherwise I would first test a simple program by sending a known command for which your instrument will respond using a VISA Open (or VISA Configure Serial Port.vi), Write, Read, and Close in series.