Why would anybody want to review papers for journals and conferences? Most people (me included) try to reduces these chores to a manageable level (by either being 'permanently unavaible', by inventing conflicts of interest, or by several other means that I won't disclose :-) (just in case there are editors reading BioStar)
But seriously, you either must live in a very different 'European country' than the ones I know of, or your research area works different from mine (what is TCS, by the way?). I have never heard of cases where paper reviewing is considered an asset for a C.V., mostly because this is done blindly and thus nobody can check how many papers you have really reviewed.
If, on the other hand, your desire is to help the scientific community, doing peer review for journals or meetings is a laudable thing. As others have pointed out, you normally don't have to do anything, the journals will find you. The usual prerequisite is that you have to be a PI (i.e. group leader) and that you have published in the field of interest (as a corresponding author).
Journals normally don't as postdocs or other non-PIs for doing reviews. However, I suspect that a large portion of paper reviews is done by postdocs, because the PI has deferred the job. The journals don't like this practise but there is not much they can do about it. Often they don't even notice because the review goes by the name of the PI.