$itemList = DataObject::get('WareItem');
$fields->addFieldToTab('Root.Content.WareItems', new CheckboxSetField('WareItems', '', $itemList));

class WarePage extends Page {

   static $many_many = array(
      "WareItems" => "WareItem",
   );
function getCMSFields() {
      $fields = parent::getCMSFields();
      $dataobjectmanager = new FileDataObjectManager(
         $this,
         'WareItems', // Source name
         'WareItem', // Source class
         'Photo', // File name on DataObject
         array('Name' => 'Name','WareBrand.Name'=>'Brand','Article' => 'Article'),
         'getCMSFields_forPopup'
      );
      $dataobjectmanager->setUploadFolder('WareHouse/'.$this->title);
      $dataobjectmanager->enableUploadDebugging();
..
$fields->addFieldToTab("Root.Content.WarePageItems", $dataobjectmanager);

      return $fields;
}
}

class WareItem extends DataObject {
...
   static $belongs_many_many = array(
      'WarePages' => 'WarePage'
   );
..
}

$dataobjectmanager = new ManyManyDataObjectManager(
         $this, // Controller
         'WareItems', // Source name
         'WareItem', // Source class
         array('Name' => 'Name','Article' => 'Article'),
         'getCMSFields_forPopup'
      );

      // SWF Upload
      SWFUploadConfig::addPostParam('holderID', $this->ID);
      SWFUploadConfig::addPostParam('holderClass', __CLASS__);
      SWFUploadConfig::addPostParam('itemClass', 'WareItem');
      $fields->addFieldToTab("Root.Content.MassiveUploader",
         new SWFUploadField(
            "EditForm",
            "Upload",
            "",
            array(
               'file_upload_limit' => 10,
               'file_queue_limit' => 10,
               'browse_button_text' => 'Add ware items ...',
               'upload_url' => Director::absoluteURL('WarePage_Controller/uploadHandler'), // Custom Handler
               'required' => 'true',
               'debug' => 'true'
            )
         )
      );