Nag123,
In the configuration dialog box of the emulator is where you specify which COM port on the computer you want to map to. As far as selecting the COM port on the actual program, you do this from the VISA resource name terminal of the VISA VI's.
If you are not very familiar with the VISA VI's I suggest you go to the Example Finder (Help » Find Examples...) and click the search tab and search for "Serial". Please take a look at the "Basic Serial Write and Read.vi" Example.
Eli S.
National Instruments
Applications Engineer