This is a way to get them without codes. But you will need the preset code on the action replay that allows you to walk through walls.
So if you want to get shaymin, turn on the walk through walls cheat and go to rout 224 which is the route behind the Pokemon league. Go to the end until you get to the white rock and walk on the water directly behind it and you will see a path. Go down the path and you will eventually reach a level 30 shaymin and you have to catch it.
For darkrai, go to fullmoon island where cresselia is and use the walk through walls cheat and then go right and walk on the water. Eventually you will reach newmoon island and this is where darkrai is. The darkrai is level 50 and you will have to catch it there.
I know this because I cheated to do this before on my pearl game so it should work. Hope I helped :)
-I beleive this is what your looking for but if it is not then just tell me and I will hide it.