Simply click on one of the smilies to the left of your reply at the time of replying. The smilie to appear for the thread overall are those options above the reply. BTW you can learn smilie codes off by heart if you recognise their text combination.
eg: : & D in combination =
(I forgot to mention, click the FAQ button above & see the bottom section which will explain how to do. It's possible to disable smilies so you may need to adjust your member posting settings.)