When you are a scorer, you can see numbers beside the title of each comment (except yours). Click on any number to score the comment to that score.
When you click on a score, you will see a small window apear, with an input field and a "submit" button. In the input field, type your opinion about the commment (interesting, off-topic, informative, etc...). You can also leave it blank. Then click on the button to proceed.