Thank´s for replay (what????).
If someone have the same problem must specify the field KEY.
I changed update_field( "evento", $eventoid, $post_created );
for update_field( "field_my_field_key", $eventoid, $post_created );

I forgot my loop:
<ul>
<?php query_posts(array( 'post_type' => 'arquivo' ) ); ?>
<?php while (have_posts()) : the_post(); ?>
<?php
/*
* Caso o campo Evento informado no Arquivo seja igual o evento
* informado via URL, busca-se todos os arquivos (mídias) a esse Arquivo
*/
$_evento = get_field('evento');
if($_evento != ""){
if($_evento == $eventoid){
$args = array(
'post_type' => 'attachment',
'numberposts' => null,
'post_status' => null,
'post_parent' => $post->ID
);
$attachments = get_posts($args);
if(count($attachments) > 0 ){
foreach ($attachments as $file){
if ( $last_id = get_post_meta($file->ID, '_edit_last', true) ) {
$last_user = get_userdata($last_id);
}
else{
$last_user = get_userdata($file->post_author);
} ?>
<li>
<?php echo date("d.m.Y", strtotime($file->post_modified)); ?>
<span>Última alteração feira por: <?php echo $last_user->display_name;?></span>
<a href="<?php echo $file->guid?>" title="<?php echo $file->post_title?>" target="_blank"><?php echo $file->post_title?></a>
<p><?php echo $file->post_content;?></p>
</li>
<?php }
}
}
} ?>
<?php endwhile;?>
</ul>
The post title, content… is show but the field ‘evento’ NOT.