Go to your Joomla backend -> components -> CComment ->settings -> new, from the component dropdown field select com_redshop and click next. Save the configuration.
Now go to Extensions -> Plug-in Manager and find the Redshop Product - CComment plugin. Enable it. Now in single view of a product you should see the comment form.
The Redshop Product - CComment belongs to the system plugin group and not to the redshop_product group.