When you hit the post or reply button, there's a link in the lower right corner that says "emoticons" ... click that and it launches a popup window with all the emoticons listed. Just copy and paste, or type, the :emoticon: of your choosing.
The emoticons that are visible on the right (above the emoticon link) are applied to the entire message ... you'll see it in the upper right corner of your post. It's confusing ... I didn't get it at first either.