Under the comment box you'll see a button that says "More Options..." (on the bottom right). Click on that and you will have... more options. One of those options will be to add code (it's an icon on the second row down from the top and second one in from the left-- looks something like this {}).
Just use bbcode. {code}Your code here{/code} replacing the {} with []. (obviously I can't post them with [] since the forums would pick them up!)