Do you have a custom menu module installed as this file is not a default magento template file from memory?
I suspect you have a third party module installed acting as a menu system and as the error suggests the template file categorymenu.phtml
does not exist in the location app/design/frontend/base/default/template/catalog/product/
.
You do not add template files to base as this is magento's default template location, you should setup a theme or simply add it to the default theme (not base) so it gets included. If this is a third party module I would re-install it, or find the missing template file and put it in the correct location, or contact the module provider for assistance.
I would also advise you read up on how Magento handles theme's: http://www.magentocommerce.com/design_guide/articles/working-with-magento-themes