Adding comments to a discussion topic (also called a thread) is very easy.
Once you are logged on find the forum, topic and message where you want to add a comment. Click on the Add New Comment link which will be at the bottom of the panel on the right hand side.
A new page will open where you must enter a title.
Type your message into the "body" section and then click on the save button at the bottom of the screen. Your message will then appear in the forum you have chosen.