What platform are you attempting to run on? The NetworkTables jar we distribute with the plugins only runs on the roboRIO. If you want to run on the desktop, you can use Maven-style dependencies to grab the desktop version. See the ScreenSteps here, you'll want the edu.wpi.first.wpilib.networktables.java:NetworkTab les:3.0.0-SNAPSHOT:desktop artifact.
http://wpilib.screenstepslive.com/s/...aven-artifacts