Open the thread that you are interested in. Once there, you have two options for posting in the thread. You can click on the orange Reply button, which brings up a box for you to write your post in, then click the Reply button underneath that box once you have finished writing your message.
The second way is to go to the end of the thread and see the area that is called Quick Reply. You'll see a line called Your message, some formating tools and a blank box. Put your cursor in the box, use a left click on the mouse, then type your message in the box and click on the button that says Post quick reply when you are ready.