Follow this link:

Scroll down to where it says "Installing the Development Plug-ins - Option 1: Online Install" and download the necessary libraries if you don't already have them on your computer.

After this is complete, right-click on the project, hover over Build Path, and choose "Add External Archives..." then navigate to user > [you] > wpilib > java > current > lib. Select these libraries: cscore.jar, cscore-sources.jar, CTRLib.jar, junit.jar, NetworkTables.jar, NetworkTables-sources.jar, WPILib.jar, WPILib-sources.jar. Then hit open. There should now be a Referenced Libraries folder under the project and you should have access to all of them in your project