How to install JPA Diagram Editor in Eclipse?
If you have Eclipse Helios, Indigo or Juno, goto Help -> Eclipse Marketplace, search for "JPA" and install the JPA Diagram Editor.
Since Eclipse Kepler goto Help -> Install New Software -> Work with: Kepler, type "JPA", search for Dali Java Persistence Tools - JPA Diagram Editor and install this plugin.
After installation right click on the persistence.xml file and choose Open Diagram, which will open this window. Now you can drag-and-drop entities here and you will see the diagram.
Beware that if you click on Delete button, you will delete the entity not only from this diagram, but also from disc! In order to erase the entity from the diagram, press this eraser button.
You can also create new entities and set their relations using this palette, but I don't use it, because it's easier to write it manually.