Underneath any post (to the right, but within each post) do you see 4 buttons: Edit/Quote/Multi-Quote & Quick Reply? (Note that the latter two dont' have captions)
If you do then just click on the one farthest to the right (Quick Reply) and the Quick Reply box beneath should get the focus, and you can enter your message.
Underneath that there's a "Post Quick Reply" button, which probably gets the most use, and a "Go Advanced" button, which is pretty useless unless you want to add emoticons or don't want to add HTML tags manually (it's mostly formatting options).
There is a "Post Reply" button on the bottom left of the thread page that's not part of the individual posts like the other buttons - That will bring up the Advanced options.
If you don't see that or any of the 4 buttons (Edit, Quote, etc.) then we'll look into it more.