This is how to assign an existing attribute to an existing attribute set

$setup = new Mage_Catalog_Model_Resource_Eav_Mysql4_Setup('core_setup');
$setup->startSetup();

$attribute_set_name = ATTRIBUTE_SET_VISIT;
$attribute_code = $attr;
$group_name = GROUP_ADIN;

$attribute_set_id=$setup->getAttributeSetId('catalog_product', $attribute_set_name);
$attribute_group_id=$setup->getAttributeGroupId('catalog_product', $attribute_set_id, $group_name);
$attribute_id=$setup->getAttributeId('catalog_product', $attribute_code);

$setup->addAttributeToSet($entityTypeId='catalog_product',$attribute_set_id, $attribute_group_id, $attribute_id);