unexpected T_OBJECT_OPERATOR

Unfortunately, it is not possible to call a method on an object just created with new before PHP 5.4. This is PHP in a version from 5.4 ongoing:

EDIT: sn_rating.php

Replace:
$post = get_Post((new BP_Activity_Activity(bp_Get_Activity_Id()))->item_id);

WITH:

$BP_Activity = new BP_Activity_Activity(bp_Get_Activity_Id());
$post = $BP_Activity->item_id();

Replace:
echo (new Rating())->generateRatingBox($post);

WITH:

$BP_Rating = new Rating();
echo $BP_Rating->generateRatingBox($post);

HOPE IT HELPS!