How can I add an AJAX callback to a checkbox?
ajax_example_autocheckboxes_callback function is not defined in the above code.

Also please update more info about what you are trying to do.

$form[$term-&gt;tid] = array(... 'ajax_example_autocheckboxes_callback' ...); //ajax callback
$form['tree']; //new field.


after the above function call you can get new built_info of $form_state as its rebuilt. So you can alter output of the field.

function ajax_example_autocheckboxes_callback($form, $form_state) {
  dsm($form_state);
  return $form['tree']; // this is a new field.
}


I will update my answer if more information is provided.