The plugin provides no UI for configuring it. Instead, you must define a PHP constant named CC_LICENSE. This is most easily done by adding a snippet like the following to your wp-config.php file:

define('CC_LICENSE', 'http://creativecommons.org/licenses/by-nc-sa/3.0/');

Questions

What exactly does this plugin add to my site?

It adds a <link rel="license" /> element to each page of your site. It also adds an appropriate element to the XML feeds (rdf, rss, atom) for your site.

Can I override the license for a specific post?

Yes, a filter named cc_license is called before returning the license. You can override that value under whatever circumstances you want.