You revive it from a Skull Fossil, like in Diamond and Platinum.
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You cannot get a Skull Fossil (Cranidos) in Pokemon HeartGold.
Must trade Cranidos appears in Diamond/Platinum (odd ID number) - Revive Skull Fossil
The card key in Pokemon Heartgold and Soulsilver is located in the Goldenrod Underground.
The Quick claw is located in the National Park in Pokemon Heartgold and Soulsilver.
You can't catch Cranidos in the wild or from a special event. You'll need to trade for it from Pokemon Diamond, Pearl, or Platinum.