Can being a reviewer for a top journal improve my chances in PhD admissions? I have got an email of invitation to review a paper in a best journal of my field. I had a paper there and somewhere else almost in that field. 

As I must invest serious time and attention to complete the review, I wanted to know if that does any advantage on improving my resumé for getting a better chance of acceptance and scholarship for my future PhD program abroad?
Basically, should I mention in my CV that I am a reviewer of that journal, when I am sending it to other universities?

P.S. I have graduated from M.Sc. two years ago.
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).