The WPILib documentation has a bunch of information on how to use the SmartDashboard, you can find it here:
http://wpilib.screenstepslive.com/s/3120/m/7932.
In a nutshell, any call to one of the SmartDashboard methods will put information in or retrieve information from NetworkTables. The SmartDashboard.put/get methods wrap this for you, so you don't have to manually search for a NetworkTables key and find the value on your own. If you have any more questions, feel free to ask.