There are two ways you can do it
<span style="font-size: 17pt">#1</span>
You need to be in the full reply screen. To get to the full reply screen you can click on the Switch to full reply button,
The red arrow is pointing to the full reply button:
Once in the full reply screen, click on the little smiley face in the menu and you'll get a drop down box of smilies (AKA graemlins). Just click on the smiley you want to insert and the code for it will appear in your post.
<span style="font-size: 17pt">#2</span>
You can use the smiley codes to enter smilies while not in the full reply screen.
For example:
Code:<div class="ubbcode-body ubbcode-pre" style="height: 34px;"><pre>

</pre>
will make
-
Code:<div class="ubbcode-body ubbcode-pre" style="height: 34px;"><pre> :doh:</pre>
will make :doh:
-
Code:<div class="ubbcode-body ubbcode-pre" style="height: 34px;"><pre> :lick: </pre>
will make :lick:
-
Code:<div class="ubbcode-body ubbcode-pre" style="height: 34px;"><pre>

</pre>
will make
and so on and so forth
To figure out what codes will bring up what smiley, in the smiley menu shown above hold your mouse over the smiley you want to know the code for and you'll see the code.