Click the blue text "Reply to [username]" that appears at the bottom-right of the post you wish to reply to.
A new box will appear showing the posted message in quotes.
Now add your reply underneath that (in the same box).
When finished, click the "Answer" button.
Alternatively, just type your contribution in the empty "Your reply" box that appears below the last post in that thread.
When finished, click "Submit your answer".