On the main forums page, click on one of the headings ("Newbies-info-introductions", "general-discussion-about-curly-hair", etc.). On the page you get, you will see the bright pink "New topic" button just above the blue band that says "Threads in Forum".
When you have clicked on a thread, the "Post Reply" button is in the same position.
Hope this helps.
Experimenting with too many products to name here. All this great info keeps my head spinning.