Go to the file {your magento root folder}/Block/Adminhtml/{module name}/Edit/Tab/Form.php add following code in top of the file
$fieldset->addField( 'followupemail_text', 'editor', array('name'=>'followupemail_text', 'label' => Mage::helper('followupemail')->__('Content'), 'title' => Mage::helper('followupemail')->__('Content'), 'required' => true, 'style' => 'height:24em;','config' => $wysiwygConfig, ));
go to the file {your magento root folder}/Block/Adminhtml/{module name}/Edit.php add the following line in this file
protected function _prepareLayout() { // Load Wysiwyg on demand and Prepare layout if (Mage::getSingleton('cms/wysiwyg_config')->isEnabled() && ($block = $this->getLayout()->getBlock('head'))) { $block->setCanLoadTinyMce(true); } parent::_prepareLayout(); }