I am trying to add a comment, and address a specific user, but his name, isn't popping out.
Is that since the site is still in beta?
the page on SE Meta doesn't mention anything about this.
The text completion to fill in @usernames will only match users who have commented on that specific answer. It will not match users participating other posts in that thread. And remember that your comments are already sent to the author of that post, so there's no need to add their name to the @user reply.
For a background about this feature, see the "Comment @username Notifications" section in