Commenting is a privilege you earn by using the site.
When you reach 50 Reputation, you earn the privilege called Comment Everywhere:
From the Privilege description:
How do I comment? Click the add
comment link under each post. Comments
cannot be multi-line, so simply press
enter to submit your comment.
Note that comments support a small
subset of Markdown, so you can do
things like italic and bold and
Note: you can always comment on every part of your own question, even at 1 reputation.
When you comment, the owner of the Post, and - if applicable - the person that's mentioned in the comment (i.e. '@Stefano') is notified via their Global Inbox.
If you are asking a question, and you want to add additional information, it's better to edit the question rather than commenting on it. This way, you'll avoid long threads of conversation, which this site really isn't designed for.
If you want to learn more about using the site, we have some brilliant Video Tutorials.