Will i still be able to capture cresselia and stuff if i use action replay on pokemon diamond?

I used action replay on my pokemon Platinum and i cant go to new moon island or whatever it called to capture cresslia or darkrai.Also what is the code for pokemon diamond where you can make any pokemon apear in the wild?Will i still be able to capture cresselia and stuff if i use action replay on pokemon diamond?
Assuming you didn't mess up your game, you should still be able to see Cresselia on that little island. However, you can't just see her, some requirements are needed to see her first. You have to obtain the National Dex, once you do that head to Canalave Town. Talk to the Sailor near the boat, once done conversing he will take you to Full Moon Island. Cresselia flies about the region so you will essentially have to chase after it.

If these requirements are fulfilled and you are unable to reach the island and see Cresselia, then your use of Action Replay essentially screwed up your game, that's what you get for cheating. You can cheat once more to get Cresselia if you find yourself in this predicament.