Arceus and darkrai

I believe that you can only get the Pokemon through cheating, or by going into a nintendo event and getting it. Those are the only ways to get them.

Im afraid you have been mis-informed because completing the national pokedex does not grant you access to darkrai or arceus. The only other ways based on the limitations you have presented are to trade with someone who cheated using an action replay or to attend a nintendo event. The only one I ever heard anything of was the event were darkrai was handed over through wireless during the release of the newest Pokemon movie. I know for sure it happened in japan, probably happened here already, but I cannot be sure. For further details may I suggest checking out the official pokemon/nintendo website? Any and all events in your area will be listed there.
Might I suggest not getting too bent out of shape over not having either of those two pokemon? The reality is you can't really get them unless you go to some serious trouble to do so. In the end they really arent all that cool.

Let's see...... If your willing to wait for the new Pokemon Ranger game, you can obtain a Darkrai legit there. Arceus on the other hand cannot. The Nat. Pokedex has nothing to do with it. Sorry.

Well, there is actually a way to get the legendary Pokemon Arceus, and the legendary Pokemon Darkrai without using an Action Replay DS. One such way to get your hands on these legendary Pokemon is to go to a Nintendo Event whenever one is being held in an area near you. Once you are there, you can receive the Pokemon through WI-FI via the Mystery Gift option. Another way that you can receive the two legendary Pokemon is by trading someone who has an Arceus, and a Darkrai that they would be willing to trade you. For news on Events, whether about an upcoming event, or just to check back on something, you should go to Pokemon.com REGULARLY, and keep nan eye on the latest news section.

Hope this helps.