Magento integrate editor Wysiwyg or magento what you see is what you get in form

In you Form.php integrate the code to get editor Wysiwyg

$fieldset->addField('field_name', 'editor', array(
'name' => 'field_name',
'label' => Mage::helper('Module_Helper_Name')->__('field_name'),
'title' => Mage::helper('Module_Helper_Name')->__('field_name'),
'style' => 'width:700px; height:500px;',
'wysiwyg' => true,
'config' => Mage::getSingleton('Module/wysiwyg_config')->getConfig(),

));

Now dont forget to add the WYSIWYG foder from the cms/model to your model folder