In a default CKAN install commenting is disabled. To enable it you have to install and enable the disqus (commenting) extension:
https://github.com/okfn/ckanext-disqus/
Please read and follow the instructions there (as well as the standard extensions documentation).
Once installed and enabled, the presence of comments on a given page is configured by your theme (see the documentation in the disqus extension for details).
In the default CKAN theme (as of v1.7), comments will be shown only on dataset and resource pages (and recent comments only on the administrative-dashboard).