If you're talking about threads, go to the section you believe your thread would fit (such as if it is a bug report, go to the bug section of the forum, etc.), and press the "*New Thread" button. You should be able to do that.
For normally replying to a thread, press the "Reply" button, and write what you want to say in the box that shows up at the bottom, then press "Submit" at the bottom of that box. It should post for you.
