Can being a reviewer for a top journal improve my chances in PhD admissions?
Yes, this helps.

Regarding the CV, I've seen frequently that people include sections like "Other scientific activity" or "Community service" where there is subsection like "Referee for:" followed by a list of journals (sometimes even with numbers indicating how many reviews have been done). So you can make this service visible in your CV.

There is another way in which reviewing may help your career: If you write a sensible review for a relevant paper it will be read by at least one editor of the journal and it's safe to assume that this editor is a respected and reputable member of the community. In this way, writing good reviews helps to build your reputation (albeit slowly as not many people see your reviews with your name attached, but at least these are the "right" people).