Drupal 7 does not catch me css file in form

I'm doing a form in Drupal 7, in a custom module created by me.

My module has a name like example_auxiliar_resource (the folder that goes into modules), but then in the .info I specify it correctly: Example Auxiliary Resource .

In a part of the form, injected html code to have a couple of fields in a div, to easily organize it with css. This fragment is in my file example_auxiliar_resource.inc

$form['offer_studies_minimun'] = array(
'#prefix' => '<div class="bloqueprim">
  <div class="estudios">',

'#suffix' => '</div>',  
'#type' => 'select',
'#title' => "Estudios mas adecuados",
'#options' => array(
  0 => t('Selecciona titulacion'),
  1 => t('E.S.O'),
  2 => t('Bachillerato'),
  3 => t('Grado medio de Informatica'),
  4 => t('Grado Superior de Informatica'),
  5 => t('Grado de Ingenieria Informatica'),
  6 => t('Master Informatica'),
),    
);


  $form['offer_studies_especial'] = array(
'#prefix' => '<div class="especialidad">',
'#suffix' => '</div></div>', 
'#type' => 'textfield', 
'#title' => t('Especialidad'),
'#required' => TRUE,
'#default_value' => !empty($form_state['values']['offer_studies_especial']) ? $form_state['values']['offer_studies_especial'] : '',
 );

The css code I want to add is this (to align them horizontally)

div{
width: 200px;
padding: 25px 0;
margin: 0;
}

.main{
background: red;
}

#bloqueprim{  /*padre*/
 overflow: hidden;
 width: 100%; 
}
#bloqueprim .estudios, #bloqueprim .especialidad{  /*hijos*/
 display: inline-block;
}

As I have seen in the documentation, there are different ways of doing it. Since I am in drupal 7, the method drupal_add_css() continues to work. But none of the methods work for me.

I've tried with:

drupal_get_path('module', 'example_auxiliar_resource') . 'css/style.css';

at the beginning of the page. (from the .inc code) I've also put it in the .info as is, but still not going.

I've also tried

$form['#attached']['css'] = array(
 drupal_get_path('module', 'example_auxiliar_resource') . 'css/style.css',    
);

inside the form, and it does not go. And a couple of other ways that have also failed me.

What am I doing wrong?