Facebook comments WebPart are not for posting on Facebook. They allow your visitors to comment on your page and are easy to use. A detailed description is here. The URL you provide for the webpart is a URL the comments are associated with. Typically the one, your visitor has in his address bar.
If you want to make a post to your facebook page, it depends on the version of kentico you are using. We've completely re-done the social features for twitter and Facebook in the latest version and there is a nice howto in our documentation.
For version 7:
You set up all the information in the settings in "social networks" category. Myke sure the settings you enter are valid for your current site. Then you are able to post using an advanced workflow or an autopost control (by default in the blog post form).