In your message you could use the smilies to the right of the message text box or the icons below will show along side the message title in the "Last Post" listing. For example; the thumb down here
and the thumb up next to the message title.
I am assuming this is what you mean.
edit: Odd... it didn't post the thumb up icon on the message list but is above the message when looking at it... wonder what I did wrong? Maybe it only works when it is the original post?