You know Earth's magnetic poles don't match up with its rotational poles right? They're kinda close, but the magnetic poles wander. Even though Mars (oddly) doesn't have strong magnetic poles, that doesn't mean it doesn't have poles. As for which one is north, we just pick a direction of the solar system to be "north". http://en.wikipedia.org/wiki/Poles_of_astronomical_bodies
I would guess that we now name the "north pole" on any planet that which your thumb points when doing the right hand rule with the rotation of the planet... but I'm not sure.
That's used for moons and dwarf planets etc. But in this solar system, we actually have a designated hemisphere that is "north" for planets. It's explained in that link I posted.
