From http://www.excellencemagentoblog.com/magento-admin-form-field Amending forms - select dropdown

fieldset_form

$fieldset->addField('select', 'select', array(
          'label'     => Mage::helper('form')->__('Select'),
          'class'     => 'required-entry',
          'required'  => true,
          'name'      => 'title',
          'onclick' => "",
          'onchange' => "",
          'value'  => '1',
          'values' => array('-1'=>'Please Select..','1' => 'Option1','2' => 'Option2', '3' => 'Option3'),
          'disabled' => false,
          'readonly' => false,
          'after_element_html' => '<small>Comments</small>',
          'tabindex' => 1
        ));
- See more at: http://www.excellencemagentoblog.com/magento-admin-form-field#sthash.DxnOQNLW.dpuf