new IMenuItemProvider() {

   @Override
   public void addMenuItem(NatTable natTable, Menu popupMenu) {
		
      final MenuItem myMenuItem = new MenuItem(popupMenu, SWT.PUSH);
      myMenuItem.setText("Some Text");
      myMenuItem.addSelectionListener(new SelectionAdapter() {
         @Override
         public void widgetSelected(SelectionEvent event){

            //Do your stuff

         }
      });
								
      popupMenu.addMenuListener(new MenuAdapter() {
         @Override
         public void menuShown(MenuEvent e) {

            myMenuItem.setEnabled(condition);
         }
      });
}