1) In the “forums” tab – either by starting a new topic – just hit the “New topic” button or by replying to someone else’s topic.
2) You can post comments on individual blog articles. Just hit the “x comments” link under each article.
Any activity on the site – whether on the forum or the blog – is automatically cross-posted on the “Activity” tab. You used to be able to reply there too – but I thought it was confusing (to me anyway) and so turned that feature off. That might be where your last comments went!
I think this is actually deserving of its own sticky thread!