$collection = Mage::getModel('productlabels/category')->getCollection();
$collection->getSelect()
        ->joinLeft(array('category' => $collection->getTable('catalog/category')), 'main_table.id_category = category.entity_id')
        ->joinLeft(array('labels' => $collection->getTable('productlabels/table_labels')), 'main_table.id_label = labels.id_label');

protected function _prepareForm()
{
    $helper = Mage::helper('productlabels');
    $model = Mage::registry('current_category');

    $form = new Varien_Data_Form(array(
                'id' => 'edit_form',
                'action' => $this->getUrl('*/*/save', array(
                    'id_entity' => $this->getRequest()->getParam('id_entity')
                )),
                'method' => 'post',
            ));

    $this->setForm($form);
    $legend = $this->getRequest()->getParam('id_entity')?'Edit label':'Create new label';
    $fieldset = $form->addFieldset('label_form', array('legend' => $helper->__($legend)));

    $fieldset->addField('name', 'label', array(
        'label' => $helper->__('Name of category'),
        'name' => 'name',
    ));

    $fieldset->addField('entity_id', 'hidden', array(
        'name' => 'id_category',
    ));

    $fieldset->addField('id_entity', 'text', array(
        'name' => 'id_entity',
        'label' => 'LABEL'
    ));

    $fieldset->addField('id_label', 'select', array(
    'label' => $helper->__('Name of category'),
    'name' => 'id_label',
    'values' => Mage::getModel('productlabels/block')->getCollection()->getTitle(),
    ));

    $form->setUseContainer(true);
    if($data = Mage::getSingleton('adminhtml/session')->getFormData()){
        $form->setValues($data);
    } else {
        $form->setValues($model->getData());
    }

    return parent::_prepareForm();
}