form_set_error checkbox CCK 2 drupal 6
Have a look if the class error has been added to your &lt;input&gt;. It has on my side, it's the CSS that does not highlight the elements properly.

You can either adjust your stylesheet for .form-item input.error or you can use a piece of jQuery to make the parent of the radios styleable, such as 

$('.form-radios:not(.error):has(.form-item input.error)').addClass('error');


Another option would be to add the appropriate class via the Drupal's theming system (theme_optionwidgets_buttons()).