Menu Call Back for Ajax not working
Your menu item looks fine so make sure you've done the usual things, make sure the module is installed and enabled, and make sure you clear Drupal's caches so the menu is rebuilt and your new item can be picked up.

On another note, if this callback is meant for AJAX you probably want to be printing something rather than returning it in your callback function. Something like this:

function callback_test() {
  print 'success';
  drupal_exit();
}