When you visit a forum page, the options available to you may depend on whether you are a member, and if so, on your membership level. If you are not already logged in, you will be prompted to sign in using your Wild Apricot credentials when you try to add or reply to a forum topic.
If you are authorized to create new forum topic, a Create topic button will appear above and below the topic list.
To reply to a topic, display the topic then click the Reply button.