My radio buttons in Drupal do not show
Is the form running through drupal_get_form? This type of error can occur when using the generic render() fuction on a form that hasn't been prepared by drupal_get_form(). Check your hook_menu settings and make sure your page callback is set to drupal_get_form() with the form id as the page arguments(encapsulated in an array)

if you must use a custom page callback, make sure that you're piping the form based output through drupal_get_form() like so...

$output = drupal_get_form('form_id');