I don't know if I can explain it right... well, when you open a thread u can see this "buttons" on the right that say 'Print', 'Send topic', 'Notify' and so on. They are, um, on a white line, above the first post on the topic. You have to click on the button that says 'Reply', and then you just type in your text in the text box and click the button that says 'Post' under the text box. =/
Hope I helped - at least a little. Welcome to the forum btw! =]