#states in add node form
You need to set the state in the "value" key, so one level deeper as you tried.
E.g.:

$form['field_yourfieldname'][LANGUAGE_NONE][0]['value']['#states'] =
  array('visible' =&gt; 
    array('input[id="edit-field-otherfieldname"]' =&gt; array('checked' =&gt; TRUE))
  );