Posting a topic means you're half way there. For future reference:

To Add A Comment:
1. Find the topic you want to comment on
2. Scroll down to the end and you'll see an ADD COMMENT button, click this
3. A text box will appear (you may need to scroll down to bottom again to see this) enter in your text and hit ADD COMMENT.

To reply to a specific post within a topic:
i. Click on REPLY TO THIS POST which appears in the blue line above the comments.
ii. make your post as described above.
iii. this will appear in grey under the post you are replying to.

Tips:
-Html will not work within posts
-Always check your posts before you hit add comment cos there's no editing
-links can be added in the link box but don't double up on the http://
- don't click the add comment button more than once even if it's running a little slow, you'll wind up adding your post twice.