I've been searching around for days but can't find the solution.. I know this is a core bug and have found two solutions on http://drupal.org but both of them don't work for multi check box validation on my site (It works for single check box and multi radio buttons though).
These are what I have found:
Required radios/checkboxes are not validated (D6)
My problem is that it's not outputting the error classes only for multi check box even after using those methods above. Any idea how to fix this?