First, you have to make sure that you have the correct type of serial cable (null modem or straight-through) and that the com settings (baud rate, parity, etc.) of the VI match what the instrument is set for. Then, you have to know whether the instrument requires a specific command before it will return anything or whether it streams data. All of this should be in the manual. Since you have not attached the manual or even mentioned the make and model of the instrument, it's impossible to provide many more details.
I would recomend that you first try a program such as Hyperterminal and get communication working there before trying any programming.
