Form alter to achieve ajax

Hello,

i implemented fook_form_profile2_form_alter to alter the registration form. I succeeded in adding some custom element validate functions i needed, but i can't find a way to add ajax to my fields. here's the code i used:

function mymodule_form_profile2_form_alter(&$form, &$form_state) {
  // Your alterations.
  /*
   * - setting user infos about card number as username
   * - hiding profile card field and giving it a function to get the username value
   */
  if((arg(0)=="card")&&(arg(1)=="register")){
    
    $form['account']['name']['#type'] = "hidden";
    $form['account']['name']['#required'] = false;
    
    $form['profile_my_profile']["field_card_card"][$form['profile_my_profile']["field_card_card"]['#language']][0]['value']['#ajax'] = array(
      'callback' => 'my_module_new_user_ajax',
      'wrapper' => 'card_div',
      'method' => 'after'
    );
    $form['profile_my_profile']["field_card_card"][$form['profile_my_profile']["field_card_card"]['#language']][0]['value']['#prefix'] = '<div id="card_div">';
    $form['profile_my_profile']["field_card_card"][$form['profile_my_profile']["field_card_card"]['#language']][0]['value']['#suffix'] = '</div>';
    $form['account']['name']['#element_validate'][]="_my_profile_copy_card_to_username";
    
    if(isset($form_state['values']['profile_my_profile']["field_card_card"][$form['profile_my_profile']["field_card_card"]['#language']][0]['value'])){
      $card_n = $form['profile_my_profile']["field_card_card"][$form['profile_my_profile']["field_card_card"]['#language']][0]['value']['#value'];
      $result = _my_module_check_card_existing($card_n);
      if($result=="KO"){
          form_set_error("profile_my_profile][field_card_card",t("Sorry, the card number you inserted is not correct."));
      }
      
    }
    
    
    $form['#validate'][] = "_my_module_new_user";
     
  }

ajax is actually added to the field, but then ajax.inc fires:

Notice: Undefined index: #ajax in ajax_form_callback() (line 380 of /home/myuser/www/myproject/includes/ajax.inc).

so i went to check there and actually the file looks for $form_state['triggering_element']['#ajax']['callback'], but that element's #ajax is not set. is it because my form alter function is not called anymore?

thanks for helping