At the top of any HubPages page, click on "explore". Then click on "forums". You'll be brought to the forum page.
If you want to reply or post a comment on an existing thread, then open the thread in question and then scroll down to the bottom where you'll see the "post a reply" button. Click on it and you can type and then post your reply.
If you want to reply directly to someone on a forum thread, just click on the "reply" link underneath their message.
If you want to start your own forum thread, then, on the forums page, look at the list of forums on the left-hand side and choose the most appropriate place where you can post.
For example, if you need help with your hub, then you can post on the "Improving your hub" forum. When you click on that link, you can see the "start a new discussion" button at the top right corner. Click on that and then you can now type your message and then submit it. Hope this helped.