Find the section of the forum your question pertains to. At the top of the section page, you'll see a dim area with the words "thread title" in it. After that, there should be a field where you can flesh out your question.
If you're just trying to message a member, click on the little mailbox up next to your username, click "start new conversation", then put their username in the "recipients" area, and then type your message.
We're all still trying to get used to this new forum software and its little idiosyncrasies.