Dmitry Khomitchuk is right. There's no need to use overly formal (and somewhat ridiculous) forms of address like "Comrade militioner". It's common to address Russian policemen the same as any other passerby, so if you need help or want to ask a question it's perfectly fine to approach them with polite "Вы". For example "Извините, вы не подскажете как пройти к ...?" If you know their names or ranks you may use them, but it's not obligatory.