First of all, you must check with your instrument's manufacturer: maybe he has a driver for LV ready to use. If this is true, your work is greatly simplified.
If not, he has to give you the communications protocol for the instruments itself (that is: parameters for opening the serial port, commands recognized by the instrument, syntax for each of them, expected response time expected for response and so on); don't be afraid of asking him: he MUST give it to you!
Once you have all these details, you can open serial port an write and read to it using vis in Instrument I/O >> Palette. I suggest you to look at the examples shipped with LV (I/O interfaces >> Serial communication) and on the NI site.
Basically serial communication is simple: write a c
ommand to the instrument, wait the appropriate time, read the answer from the instrument. The problem is to speak and listen with the correct language (the protocol I said before).
Sorry if my response seems too general: the true fact is that 'serial communication' is quite too general a matter: once you have defined your instrument requirement, we will be able to orient you in the correct direction.
Roberto
