When you replied to the original message you probably saw a bunch of smilies to the right of where you typed your message. Just click those to add them to your message. There are more available by clicking the
More in the bottom left of the smiley list.
If you are asking how to add them to your signature (posted to the bottom of every message) you need to click
User CP up near the top left of the screen, scroll down a bit and click
Edit Signature on the left. Add smilies and save!
Hope that helps.