Are you referring to the kind of smiley you see at the top of this post? If so, at the bottom of where you're composing your message/post, there should be a little box that says "Post Icons" with a list of 14 different icons and smilies you can pick from. Just click or tap on the dot to the left of the one you want (so that it looks like a filled-in standardized test bubble) and it should appear at the top of your post or private message.
When replying to a private message, as far as I can tell, it only pops up when you select the "Go Advanced" option at the bottom. When you go to send an entirely new message (one that's not a reply to a message thread already in progress), it should already be there at the bottom.
You should also be able to use the "Preview Post" function to see how it looks before you post.
Hope that helps!
