Its easy!
To start a conversation, first click on the dark blue "Questions and Comments" in the menu path above to get back to the forum page. Next, click on the red "New Topic" in the upper left and then write your question or comment. Be sure to click "Submit" at the bottom of the page to post your comment into the conversation.
To reply to an ongoing conversation, click the red "Post Reply" in the upper left, write your comment, and click "Submit" below.