How to add a new variable

I am trying to add an new variable in the the product page that pulls in information from a new field the manufacturers tables.

I have created a new field manufacturer_desc and wish to show this underneath the description on the product page.

the idea being each product has the manufacturers desction they are assigned to. But I keep geeting and undefined variable error.

I have added this line to the product.pho file
$this->data['manufacturer_desc'] = $product_info['manufacturer_desc'];

And this to the product.tpl file
<?php if ($manufacturer) { ?>
<?php echo $manufacturer_desc; ?>
<?php } ?>

Anyone any ideas how i can resolve this.
thanks
chris

You need to edit the model file to make sure that it is fetching the manufacturer_desc data from the database and putting it into the $product_info array