Hi RayPh,
Are you able to communicate with the instrument using other application software such as HyperTerminal?
If so, check the settings under properties.
In Labview: At the Block Diagram, select Instrument IO from the Functions Palette. Select Serial and choose "VISA Configure Serial Port". Wire the configuration settings from the application software to this vi.
The Context Help will show a description of each terminal and their default settings.
You can also check the following link for more details:
http://exchange.ni.com/servlet/ProcessRequest?RHIVEID=101&RPAGEID=137&HOID=5065000000050000008E490100&HTHREAD=000084366&UCATEGORY_0=_49_%24_6_&UCATEGORY_S=0