Can't add JUnit 4 library to the build path in Eclipse

I have some trouble with adding the JUnit 4 library to the build path (using Eclipse). I have these imports:

import static org.junit.Assert.*;

import org.junit.Before;
import org.junit.Test;

This error occurs: "The import org.junit cannot be resolved". I already found out why it's saying that.

I've tried different ways to add the library, but I always get this error:

What is causing this? And how can I fix it?