I've helped our team often with this error. The problem is the plugin hasn't set up the necessary environment variables to tell the project where WPILib jars are.
To fix:
1. Go into Eclipse > Preferences.
2. Go into Java >Build Path >Classpath Variables.
3. Click New.
4. Enter Name: wpilib.
5. Click File... and navigate to <user directory>/wpilib/java/current/lib/WPILib.jar
6. Click OK.
7. Repeat steps 3-6 for:
Name: networktables, <user directory>/wpilib/java/current/lib/NetworkTables.jar
Name: wpilib.sources, <user directory>/wpilib/java/current/lib/WPILib-sources.jar
Name: networktables.sources, <user directory>/wpilib/java/current/lib/NetworkTables-sources.jar
I'm assuming you've installed the WPILib Eclipse plugin. If you haven't, please do so by:
1. Go to menu item Help > Install New Software...
2. Click Add...
3. Enter
http://first.wpi.edu/FRC/roborio/release/eclipse/ for Location and click OK.
4. Check the checkbox next to WPILib Robot Development and click Next at the bottom until you finish.