The lists in the left-hand column are all focused on navigating through the Boards, Folders and Discussions. Listbox navigation can be done by single clicking on the Board, Folder or Discussion you want; or use the arrows to move sequentially through Folders or Discussions. To go back to the Main Boards screen, click on Boards Menu/Options.

Use the navigation buttons that appear when you read a Discussion for other tasks:
Use the New button to post a new Message.
Use the Reply to All button to reply to all messages in the current Discussion.
Use the Reply button to reply to the current Message.
Use the small arrows to navigate through the Discussion.