My professor sometimes asks me to referee papers for him. Should I list the corresponding journals in the professionals activity section of my CV?
Yes, if your professor explicitly lists you as a reviewer or sub-reviewer with the program committee of the conference/journal; no if your review is then edited by your professor before submission.
Yes. It is the very prototype of service to the professional community that you want to show that you are willing to perform.
The question that Scrooge linked to has good advice for how to do so.