How to populate node reference select

Hi,

I need to populate a CCK node reference select in a form when it's showed.

I need to show only the nodes that have the status=1. So, I select the Event "T content A node form is being built", the Action "Execute custom PHP code", I created an array ($options) with the nid and the title of each valid node ($options[$nid] = $title;) and I put it in $form['field_name']['#columns'] = $options;

but it doesn't work...what am I doing wrong??...

Thanks!