Done.
1) Download the sources, from "http://java.sun.com/javase/downloads/index.jsp" choose "Java SE 6 JDK Source Code".
2) Extract the sources: "java -jar jdk-6u1-ea-src-b03-jrl-19_jan_2007.jar"
3) From Eclipse, choose the command Preferences->Java->Installed JREs
4) Edit the default JRE
5) Source attachment...
6) External folder -> j2se/src