Commenting is easy peasy, here is what you do: Under the post on which you would like to comment, click on the link where that says "0 comments". If there are comments to the post already it may say "1 comment" or whatever the number of comments is.
Clicking on the link will take you to the commentbox:
Write your input and choose a profile (no username, codes etc.). Some use their Google account but if you don't have one you can use "Name/URL" (with this one you just enter your name, a nickname or whatever you fancy). Or you can use "Anonymous". Press "Publish" and you're done!
Don't panic if you don't see your comment right away! I am to publish it finally by an approval link I get as soon as the post ticks in. This is to avoid weirdos posting nasty things, or spam. Once I've approved the comment you and everyone else can see it.