How can I write an input selector for the states API?
drupal_process_states() has a great set of information on handling inputs for different types of input elements. 

You have checkboxes, so you have to use checked assertion. 

case 'my_form':
    $form['revision_information']['#access'] = FALSE;
    $form['field_guide_link_hosting']['#states'] = array(
      'visible' =&gt; array(
        ':input[name="field_guide_types[und][s]"]' =&gt; array('checked' =&gt; TRUE),
      ),
      'required' =&gt; array(
        ':input[name="field_guide_types[und][s]"]' =&gt; array('checked' =&gt; TRUE
      ),
    );


Notice I replaced the value assertion with a checked, and also changed the jQuery selector to use the full name attribute of the checkbox.