ah, well, the MyWorld.class ended up in the root directory again. Whereas in the Manifest, it is declared to be "Hello.MyWorld", so the kindle would search for "Hello/MyWorld.class". I'm just guessing here, but you might just have issued "jar cvfm [...] Hello/MyWorld.class", but it should have been "jar cvfm [...] Hello".
You can check the contents of the .azw2 with a ZIP reader to make sure that the layout within matches the data in the Manifest.
Edit: or just use the jar tool's "t" command...