The code I've created does get the correct output, but won't write it in the Event node display screen.

The code:

function agenda_nodeapi($node, $op, $arg = 0) {
  global $form_values;
  drupal_set_message(" agenda_nodeapi(..., $op, ...)");
  switch ($op) {
    case 'view':
        drupal_set_message("agenda_nodeapi: viewing node");
        // If the user has the view agenda perm and this node is agenda enabled
        // display the agenda list.
        if (user_access('view agenda') && agenda_type_is_enabled($node->type)) {
          drupal_set_message("agenda_nodeapi: add agenda list");
          $output = agenda_list($node->nid); // This gets a themed table.
         drupal_set_message("agenda_nodeapi: output is $output");
          // Save output into a node property for retrieval from the theme layer.
          $node->agenda_view = $output;
          // Store the data directly into the content array, for default display.
          $node->content['agenda'] = array(
            '#value' => $output,
            '#weight' => 15,
          );
        }
      break;