The <junit> Ant task is an optional task and this is the standard Ant error message when Ant cannot find the supporting code for an optional task. Ant includes support for the tasks themselves, but not the internal code that actually does the work.

So you need to find a copy of junit.jar, the code that does JUnit work, and tell Ant where it is. The easiest way to do this is to go to Window --> Preferences, Ant --> Runtime. In the Classpath tab click on Global Entries and then Add External JARs....

Eclipse already has a copy of JUnit, so in the dialog find your Eclipse plugins directory ([ECLIPSE_DIR]/plugins/) and go to the JUnit plugin (presently org.junit_3.8.1) and select junit.jar. Now all of your Ant scripts will know how to do the <junit> Ant task.