It would be very helpful if you could open the "Error Log" view and find the stack trace related to the NullPointerException. Please paste the entire stack trace into this thread so we can get more info on this exception. Thanks.
Oracle Enterprise Pack for Eclipse 188.8.131.52 is available for Eclipse Juno (3.8 and 4.2).
If already the 184.108.40.206 version Could not write to persistence.xml file. java.lang.NullPointerException indicates persistence.xml is not found. Does the JPA project created have the persistence.xml file, eventhough an error has been generated? If JPA project does not get created, create an EJB project and add the JPA project facet to the project.