Perhaps JAVA_HOME does not point to the JDK...

Buildfile: C:\dev\EJB3Tutorial\EJB3JSPExample\ejbModule\build.xml
init:
build:
[javac] Compiling 2 source files to C:\dev\EJB3Tutorial\EJB3JSPExample\ejbModule\build\classes

BUILD FAILED
C:\dev\EJB3Tutorial\EJB3JSPExample\ejbModule\build.xml:41: Unable to find a javac compiler;
com.sun.tools.javac.Main is not on the classpath.
Perhaps JAVA_HOME does not point to the JDK.
It is currently set to "C:\Program Files\Java\jre6"

Total time: 219 milliseconds

A solution to this in Eclipse is to edit Ant's runtime properties (Window > Preferences > Ant > Runtime). Select the Global Properties node in the Classpath tab and click Add External Jars. Select tools.jar from your JDK directory (e.g., C:\Program Files\Java\jdk1.6.0_20\lib\tools.jar).