meta-box.php

<?php

add_action( 'admin_menu', 'mixpanel_create_meta_box' );
add_action( 'save_post', 'mixpanel_update_event_label' );

function mixpanel_create_meta_box(){
  if( function_exists('add_meta_box') ){
    add_meta_box( 'mixpanel-event-label', 'MixPanel Event Label', 'mixpanel_event_box', 'page' );
  }
}

function mixpanel_event_box(){
  global $post;
  $mixpanel_event_label = get_post_meta( $post->ID, 'mixpanel_event_label', true );
  ?>
  <table class="form_table">
    <tr>
      <th width="30%"><label for="mixpanel_event_label">MixPanel Event Name</label></th>
      <td width="70%"><input type="text" size="60" name="mixpanel_event_label" value="<?php echo $mixpanel_event_label; ?>" /></td>
    </tr>
  </table>
  <?php
}

function mixpanel_update_event_label( $post_id ){
  if( isset($_POST['mixpanel_event_label']) ){
    update_post_meta( $post_id, 'mixpanel_event_label', $_POST['mixpanel_event_label'] );
  }
}
?>