By buying the action replay game card.
No Yes and you can find action replay codes for the action replay for heartgold ans soulsilver all over the internet.
You cant even with action replay.
There's no Action Replay code for Zoroark in Pokemon HeartGold, because Zoroark does not exist in Pokemon HeartGold.
There is not a known Darkrai Action Replay Code for Pokemon HeartGold and SoulSilver yet.
get action replay
yes at supercheats.com/pokemon heartgold action replay codes
you cant for heartgold or soulsilver suckers
yes. You can use the action replay in any Pokemon game.
event or action replay
Yes
By Action Replay
you have to hook the replay up on the computer and download it.