Ok, I'm using it on my iPhone so I'm not sure if it's the same on all. You had to go to the "Forum" button at the bottom. That will then list all the forums, choose the one you want to post in (and the subdivision if there is one). Then you will see an arrow in a box at the top right (same as what you use to reply) and there you can pick "post new topic".
The following are two pic that will hopefully help. The first is showing the "forum" button highlighted and the list of forums. The second shows the arrow thingy at the top. Hope that helped!!!