if you want emoticons like this ------> then you need to write : icon emoticonname plz : but without the spaces. You need to know the emoticon name though and an easy way to do that is just move your mouse over the pic, the name will pop up. examples:
if you want regular dA icons like these -----> you just have to type in regular faces like you would anywhere else or to make your life easier, click on "Emoticons" at the bottom of the comment box and it'll give you a little handy-dandy box with every deviantART emoticon, with an example on the right and it's code that you would type to use it on the left.