The WODisplayGroup class defines the actions insert and delete, which you'll bind to the Insert/New and Delete buttons. It doesn't, however, provide a save method. You'll have to provide that yourself.
public void saveChanges() throws Exception {
try {
this.session().defaultEditingContext().saveChanges();
}
catch (Exception exception) {
System.err.println("Cannot save changes ");
throw exception;
}
}