posting a messge related to an existing post
Click the reply button on the post you wish to reply to. Note that if a bunch of people reply to the same post, the first reply will migrate down the list. Hope that helps.
ETA: Hmmm, maybe my second statement is false. Sorry, now I'm all caught up in testing this theory.
The software has a way of ordering replies so that it tries to put a reply immediately under the post to which it is replying, unless there are intervening replies to intervening posts, as in the example by "small h" above. The best you can do is to click on "reply" on the post to which you're replying, and the software will usually put it directly below that post, or as close as possible while allowing for intervening replies.