As you're creating the post, you click on whichever smiley you want from the right side grouping of smilies. It will be placed at the current location of the text cursor (the bar cursor). Just be sure you have left the text cursor in the text window or it won't work.
Or you can type in the simple codes for the various smilies if you remember them or have a list of them.
:D is the big smile smiley
:) is the "normal" smiley