To comment on an entry, just type your comment in the box provided and press Enter (this doesn't reload the page, so you can just continue reading your feed from your previous position). HTML, images, videos, and mentioning other LJ users by their usernames are allowed in quick comments. You can also edit or delete the quick comment you left directly from the Friends Feed.
This feature is available in the new design and new Feed only.
You can switch to the new design by clicking the link in the upper right page corner, and the Friends Feed is available at http://username.livejournal.com/fee