The import org.junit cannot be resolved

You need to add junit library to the classpath of your project. There are several choices to achieve it depending on your development setup.

1. Command line: In the case of command line invocations, you will have to add junit.jar to the classpath of your application with java -cp /path/to/junit.jar. Take a look at the answers here.
2. Using eclipse: Eclipse distributions are bundled with this library and this is how you can use it for your project. Right click on the eclipse project and navigate:

Properties -> Java Build Path -> Libraries -> Add Library -> JUnit -> Junit 3/4

In the scenarios where you want to use a different version of the jar, instead of clicking on Add Library above you should click on Add External Jar and locate the library on the file system.