unable to locate the Javac Compiler in:   
C:\Program Files\Java\jre6\..\lib\tools.jar Please ensure you are using JDK 1.4 or above and not a JRE 
(the com.sun.tools.javac.Main class is required). In most cases you can change the location of your Java 
installation by setting the JAVA_HOME environment variable.


I fixed the problem by changing the used JRE to the one inside the JDK. 

Go to Preferences/Java/Installed JREs and add one for the  location "C:\Program Files\Java\jdk1.6.0_27\jre" or something like that. 
Remove the one for "C:\Program Files\Java\jre6". 

As you can see, the path "C:\Program  Files\Java\jre6\..\lib\tools.jar" only makes sense if the first part  (til the /..) is replaced 
by "C:\Program Files\Java\jdk1.6.0_27\jre".