Add WYSIWYG editor to admin custom field

To enable the editor for a certain editable textfield, just use 'wysiwyg' => true, instead of 'wysiwyg' => false. and add 'config' => Mage::getSingleton('cms/wysiwyg_config')->getConfig(). i.e.:

$fieldset->addField('description', 'editor', array(
'name' => 'description',
'label' => Mage::helper('sevents')->__('Description'),
'title' => Mage::helper('sevents')->__('Description'),
'style' => 'height:12em;width:500px;',
'wysiwyg' => false,
'required' => true,
));

After Change

$fieldset->addField('description', 'editor', array(
'name' => 'description',
'label' => Mage::helper('sevents')->__('Description'),
'title' => Mage::helper('sevents')->__('Description'),
'style' => 'height:12em;width:500px;',
'wysiwyg' => true,
'required' => true,
'config' => Mage::getSingleton('cms/wysiwyg_config')->getConfig(),
));

also add this to your controller initAction() before return.

if (Mage::getSingleton('cms/wysiwyg_config')->isEnabled()) {
 $this->getLayout()->getBlock('head')->setCanLoadTinyMce(true);
}

That's all done.