If I'm interpreting the OP correctly, they are already a medical school grad trying to match as an IMG. To that scenario I would suggest that a psychology masters isn't a total waste of time, but one should make sure that it demonstrates (via LORs, publications possibly) that one is able to work effectively in the clinical environment--e.g. a masters project that involves patient interviews perhaps, or has immediate relevance to a health care context. This *might* give you a leg up on run of the mill IMGs, though probably the most important thing you can do to strengthen the application is pass your USMLE 3.