$wpdb->insert not working with wordpress plugin i am creating

Everything seems to be working fine until i get to the insert. I am trying to get this to work on a plugin i am creating. This is for the plugin options section.

Here is the form code:

<form id="mcp_form_submit" action="<?php echo admin_url(); ?>/admin-ajax.php">
<?php 
 ?>
<?php $spages = get_select_pages(); ?>
<?php //do_settings_sections( 'mcp-settings-group' ); ?>
<table class="form-table">
    <tr valign="top">
    <th scope="row">Page Title</th>
    <td>
    <select id="select_dropdown" name="page_title">
    <?php foreach ($spages as $spage) {?>
    <?php //check to see if it hasnt already been edited. make function to check ?>
      <option value="<?php echo $spage->ID; ?>"><?php echo $spage->post_name; ?></option>
    <?php } ?>  
    </select>
    </td>
    </tr>

    <tr valign="top">
    <th scope="row">Meta Description</th></td>
    <td><textarea id='metadesc' placeholder="(Max 160 Chars.)" rows="4" cols="50" name="meta_description" value=""></textarea>
    </tr>
</table>

<?php submit_button(); ?>

And here is the form submission code:

function mcp_insert_custom_table_do($post_id, $desc)
{
    global $wpdb;
    //check to see if it exists
    $rw = mcp_get_by_id($post_id);
    if ($rw == NULL) {
        $res = $wpdb->insert( 
        'wp_meta_changer', 
        array( 
            'post_id' => $post_id, 
            'description' => $desc 
        ), 
        array( 
            '%d', 
            '%s' 
        ) 
    );

    return $res;    
    }
    else 
    {
        //Update it
        $res = $wpdb->update( 
            'wp_meta_changer', 
            array( 
                'description' => $desc
            ), 
            array( 'post_id' => $post_id ), 
            array( 
                '%s'
            ), 
            array( '%d' ) 
        );
        return $res;
    }    
}

It works by AJAX, and that part is working. It’s only the actual $wpdb->insert that is having a problem.