As far as I know there is no post requirement to like or thank a post, though I might be wrong about that.
However you can only thank/like/dislike the very first post in a thread. None other. Secondly you can only see those buttons in other people's threads, so the thread maker of this thread can't see those buttons in his own threads.
The thanks/like/dislike buttons are located in the left bottom corner of the users posts, even lower than his signature and right below the users avatar, reputation points etc. It's on the same light grey bar where you can find the quote/multi quote/edit etc. buttons, only those are on the right side, while these are on the left side. All the way on the left you got a smiley icon for thanks, then a thumbs up icon to like and then a thumbs down icon to dislike.