Add the following code to ProductController.php @ /controllers/front/ProductController.php
$id_lang = $this->context->language->id;
$manufacturer = new Manufacturer($this->product->id_manufacturer, $id_lang);
$manuf_description = $manufacturer->description;
$this->context->smarty->assign('manufact_desc', $manuf_description);
Then Add the variable $manufact_desc to the product.tpl template.
Watch out with the quotation formal.
http://www.prestashop.com/forums/topic/305984-add-manufacturer-description-into-product-page/