Access in anonymous user

I fixed it by adding few lines :

function campaignmonitor_user_form() {
global $user;
// Replace api_key and list_id with your own details
$api_key = variable_get('campaignmonitor_api_key', '');
$list_id = variable_get('campaignmonitor_list_id', '');
$email = $user->mail;
$arg2 = arg(2);
if((!$user->uid && $arg2 == 'register') || ($user->uid)){
if (_campaignmonitor_is_subscribed($api_key, $list_id, $email, TRUE)) {
$default = TRUE;
}
else {
$default = FALSE;
}
$form['subscribe_newsletter'] = array(
'#type' => 'checkbox',
'#title' => variable_get('campaignmonitor_userpagedisplaytext', 'Newsletter'),
'#default_value' => $default,
);
$form['is_subscribed'] = array(
'#type' => 'hidden',
'#default_value' => $default,
);
$form['submit'] = array('#type' => 'submit', '#value' => t('Save'));
return $form;
}else{
drupal_goto('my_url');
}
}