Is it OK for an MSc Student to Review a Scientific Paper?
This is a good question: at what point in one's academic career should one begin refereeing papers?

I think the first order of business is to make sure that the editor knows you are an MSc student.  In fact, inquiring into why you were chosen to referee the paper seems reasonable to me: the answer may help you determine whether you are qualified.  Perhaps for instance your thesis advisor got the request and passed it along to you.  That's a good situation for you: you can read the paper for correctness (in my experience, assuming the requisite base level of competence and understanding, the younger the referee, the more likely she is to conscientiously read and check a paper for correctness) and then solicit your advisor's help in determining the appropriateness for the journal.

In fact, no matter what this is a good opportunity to talk to your advisor: she will be the best person (aside from you) to help you determine whether or not you are "ready" to competently referee the paper.  If she says no, you should probably turn down the request.  If she says yes, see if you can get her help on the higher level issues that you are rightly concerned about.

Let me also say that you have to start refereeing papers sometime (or you become someone who never referees papers even into the later stages of their career: I know such people, and although so far as I know they landed in that situation through no fault of their own, it is clearly an undesirable state of affairs for the community at large), and no matter what age or rank you start, you will still have to wrestle with the issues of knowing what standards to impose.  (For that matter, sometimes I get a referee request from a journal that I have never read or even heard of before.  I try to ask the editor for more information but have sometimes just been told things which amount to "Use your best judgment."  So I did.) There is a lot of subjectivity in the refereeing process, and though you may feel less confident about your opinion as a very junior academic, in reality it is far from clear that what you do will be worse than what some more experienced person would do!

Shorter Version: You need to get a sense of whether this is a job that you can handle competently in a reasonable amount of time.  Don't be afraid to ask for guidance in determining the answer to this.   Being a master's student does not disqualify you in any obvious way.  If you can do it -- without interfering too much with your other responsibilities, of course -- then you probably should.  It will be a valuable learning experience.