Tap thepen and paper icon at the top right on your Newsfeed to create a post. To mention a user, add @ and their username. Then, she will be notified of your post.
Click the comment icon at the bottom of the post. To mention the poster, add @ and her name will pop up. Or add a few letters and scroll through the usernames that appear above the comment box and click the username you’d like to mention. The user will then be notified of your comment.