Here is what I did:
It still says that "Source not found" and I get the same old...
What could have gone wrong?
If you can point me to a wiki page or something that would be fine since the plugin's wiki is empty and googling for it did not help either.
I also installed JadClipse for Eclipse 4.x 4.0.0. I'm using Kepler SR1.
The .class file I'd previously opened still remained "Source not found" so I right clicked on the class in the Project Explorer and selected Open Class With > Jad (Ctrl_Shift-V)
That worked for me. The class file was in a jar referenced in one of my eclipse projects.
If you didn't get the Open Class With > Jad menu then perhaps you should try uninstalling and re-installing?
Hope that helps.