For the simple ones you just put {balloon}text{/balloon} in your post but use square brackets instead of curly ones.
If you want to move the balloon thing across a bit to line up with the smiley you can put, say, {pad}20{/pad} in front of the balloon to move it across 20 pixels or whatever. Again use the square brackets.
There are some more complicated ways of doing it that let you move the 'tail' on the balloon and alter the text size. You can always 'quote' somebody else's post to see the 'code' that produced it - and then back out of actually posting the quoted text.