Sorry but Sitefinity "Comments" are not available for Ecommerce products. The Ecommerce CatalogManager implements a different interface from the manager classes used by "content" such as news and blogs that do implement commenting.
Rating and reviews (which would include the concept of "comments") is already in the list of additional functionality being considered for the Ecommerce module but currently there is no timeline for when this feature will be available.
I have added your suggestion as a feature request. You can track and vote for this at:
In the mean time you might want to look at a third party tools such as Disqus:
The Sitefinity marketplace contains a control that already has Disqus integrated into it:
(see Release notes: 5.2.3700.0
the Telerik team
Do you want to have your say in the Sitefinity development roadmap? Do you want to know when a feature you requested is added or when a bug fixed? Explore the Telerik Public Issue Tracking
system and vote to affect the priority of the items