Attaching a system js library using #attached property (D7)
Going from the working example, I imagine this is what you need:

$form['#attached']['library'][] = array('system', 'ui.droppable');


Without the module name Drupal won't know where to find the library hooks.

But if you want to add a custom library from sites/all/libraries/myAwesomeLibrary this is the right way:

$form['#attached']['libraries_load'][] = array('myAwesomeLibrary');