Some classes are not set up to have a discussion section to click to find discussion questions. If this is the case, go to the home area and find the discussion in the list there and click on the discussion.

For example, here is a discussion question in the class css106.

Once you click on the discussion click on the reply button to reply to the discussion. This reply to the main question and not individual users. You can either type in the text box that shows up or upload a document.