Using onChange with CakePHP

javascript - if I can get the code to just fire the alert I have defined I would be happy.
 
<script language="javascript">
function showFields(){    
    var selected = document.getElementById('freq_type').value
         alert(selected);
    // Hide all fields we show/hide just to be sure we don't show the wrong ones.
    document.getElementById('div_freq').style.display="block"
    document.getElementById('d').style.display="none"
    document.getElementById('e').style.display="none"
 
    if(selBox.selectedIndex==0){
    // Show the fields we want to show if the first option is selected.
        document.getElementById('c').style.display="block"
    }else{
    // Extend with more else..if-statements or use a switch-statement......
    }
}
</script>
 
CakePHP view  code
 
<div class="rssfeeds_form"> 
<?php echo $form->create('Tweetlater');?>
    <fieldset>
         <legend class="legend_add_rssfeeds"><?php __('Add a Scheduled Tweet');?></legend>
    <?php
        echo $form->input('userid', array('type' => 'hidden', 'value' => $UserID));
        echo $form->input('name', array('label' => 'Description: ', 'size' => '30')); 
        $options_postto=array('T'=>'Twitter','P'=>'PingFM');
        echo "Post To: ".$form->select('postto',$options_postto,'T');
        echo " ";
        $options_freq_type=array('H'=>'Hour','D'=>'Day','N'=>'Now');
        echo "Frequency Type: ".$form->select('freq_type',$options_freq_type,'H', array('onChange'=>'showFields(this)'));        
        echo " ";        
        $options_freq=array('1'=>'Every Hour','2'=>'Every 2 Hours','3'=>'Every 3 Hours','6'=>'Every 6 Hours','8'=>'Every 8 Hours','12'=>'Every 12 Hours');
        echo "Frequency: ".$form->select('freq',$options_freq,'1', array('showEmpty'=>'false'));
        echo " ";
        $options_num_posts = array('0'=>'0','1'=>'1', '2'=>'2', '3'=>'3', '4'=>'4', '5'=>'5');
        echo "Number of Posts: ".$form->select('numposts',$options_num_posts,'0');
        echo "<br />";
    ?>
    </fieldset>
<center><?php echo $form->end('Submit'); ?></center>
</div>