Override isValid in your form

 

 /**
 * Validate the form, check passwords.
 *
 * @param array $data
 * @return boolean
 */
 public function isValid($data) {
 	$valid = parent::isValid($data);
 	 if ($this->getValue('password') !== $this->getValue('password2')) {
 			$valid = false;
 			$this->password2->addError('Passwords don\'t match.');
 		}
 
 		return $valid;
 }