You can attach a source archive (another JAR file which include the source code) or source folder to this library. Eclipse will then use this resource to show the source instead of a the decompiled code. Setting the source attachment also allows source level stepping with the debugger.

please refer to Eclipse help:

http://help.eclipse.org/ganymede/topic/org.eclipse.jdt.doc.user/reference/ref-properties-source-attachment.htm

The source JAR files of ZK Package is under the dist/src folder in the already extracted ZK package.

Please refer to Eclipse help:

http://help.eclipse.org/ganymede/topic/org.eclipse.jdt.doc.user/reference/ref-dialog-javadoc-location.htm