All you have to do is single click the face you want. When you do, you won't see it in your post right away, that might be what is confusing you. You will only see a code which represents the face, like : ) or :grin:.
Press "Preview" to see the real faces, or press "Post" when you are done and you will see the real faces then.
You can click "[more]" and a new page will appear with even more smiley faces. Single click any of those you want, then close that window to get back to your post.