Prerequisities to become an MD-PhD? Officially, I am studying computer science. However, what I am actually doing is computational neuroscience. More over, the research I do is mostly medical research.

I have noticed that there are programs for MD-PhDs. However, in most countries, there seems to be always some legal requirements one has to fulfill to enter such programs. These requirements are not given very clearly out in most cases.

With an M.Sc in computer science, can I apply to such programs? More trickier question is would it be wise..? I mean, I could just go to a neuroscience program. However, I would be also really interested to help people with psychological, neurological, etc. issues (work in hospital), when having the MD title is a must?
Most of the "legal requirements" associated with MD-PhD programs are financing related. In general, such programs are usually sponsored and funded by national health ministries (or their equivalents). Therefore, it is preferred (or required) only to have legally qualified citizens participate in such programs. 

As for the choice of major, in general, computer science is not one of the usual entry points for MD-PhD programs, which tend to be more closely related to experimental programs and majors (e.g., chemistry, biochemistry, biology, bioengineering). That said, there is more of an emphasis nowadays on computational science in medicine, and more and more MD-PhD programs are expanding their scope to include bioinformatics and computational biology. But it is still definitely not a universal, and CS majors will be at something of a disadvantage.