Hi,
I finally got my code to work.
I had <input type="submit" value="find"> in the wrong spot in head_admin.tpl. However if anyone is interested, I had to remove the reference to (int) in the product_modify.php file. For me it was not appending the productid to the url so it couldn't find the product. My php select statement looks like this now:-
$category_query = db_query("SELECT productid FROM xcart_products WHERE productcode = '" . $product_code . "'");