Depending on which Eclipse package you downloaded you may already have the source. Look in Help > About Eclipse (Eclipse > About Eclipse on Macs) and click Installation Details. In the 'Installed Software' tab check for 'Eclipse SDK' - if you have this the source is installed.
If you don't have the source you can install it from the SDK update site for your Release - http://download.eclipse.org/eclipse/updates/4.3 for Eclipse 4.3 (Kepler). Install Eclipse SDK.
Once you have the source installed go to Preferences > Plug-in Development and check the Include all plug-ins from target in Java search option (this is assuming you do not have a separate target platform). Once this is set searches for Java classes and menu options such as Navigate > Open Type will find classes in the Eclipse source as well as your source.
The Java editor is in the org.eclipse.jdt.ui plugin - org.eclipse.jdt.internal.ui.javaeditor.CompilationUnitEditor. Be warned that this is very complex code!