Should I mention that I refereed a paper in my CV/SOP?
Yes, it can be mentioned. I would put a category on your CV titled "service to the profession." In that section, mention the year and the name of the journal that asked you to referee. It's not a major point, but it shows that you're willing to be a team player and help out the journal. That speaks both to your professionalism and to the fact that the editor trusted your judgment.