You shouldn't need to edit func.product.php.
To add a new field to your product add/modify page (without using x-cart's built in "Extra Fields", you neeed to:
1) Add the new database field to the products table.
2) Modify skin1/main/product_details.tpl in order to put the field on the template for editing.
3) Modify include/product_modify.php in the section:
Code:
$query_data = array(
Step 3 is likely what you are missing, which is necessary to save the field to the database. It should then show up in your product array by default.