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I found the issue. You cannot assign fields if there are not published (in fact there are assigned, but the details view in not correct.

This will be included in next version

but if you want to fix that:
edit administrator/com_adsmanager/models/field.php
and replace beginning of function getFieldsbyPositions by :

function getFieldsbyPositions($published = true) {
if ($published) {
$sqlpublished = "AND f.published = 1 ";
} else {
$sqlpublished = "";
}
$this->_db->setQuery( "SELECT f.* FROM #__adsmanager_fields AS f ".
"WHERE f.pos != -1 $sqlpublished ORDER by f.posorder" );

then edit administrator/com_adsmanager/views/admin/view.html.php
replace
$fDisplay = $model->getFieldsbyPositions();
by
$fDisplay = $model->getFieldsbyPositions(false);