Lylirra made a [blue post][1] about drop chances.  You have to have 5 stacks of Nephalim Valor and the drop chance scales up to 100% with monster power level.

> You'll need five stacks of NV in order for the keys and the demonic organ pieces to drop.
>
>Currently, both the keys and the demonic organ pieces (Vengeful Eye, Devil's Fang, and Writhing Spine) have higher chances to drop in higher MP levels, all the way up to 100% in MP10.

When they appear, they'll show us as legendary items, making them easy to distinguish from the other items the mobs may pop.

[Diablofans][2] posted information on the location of all of the keys:

> - Key 1 - Found on Goat Mutant Key Warden in the Fields of Misery (Act 1)
> - Key 2 - Found on Dune Dervish Key Warden in Dahlgur Oasis (Act 2)
> - Key 3 - Found on Morlu Spellcaster Key Warden in Stonefort (Act 3)
> - Infernal Machine Plan - Found on Terror Demon Key Warden in Silver Spire Lvl 1 (Act 4)
> - The Key Wardens will always spawn in each Inferno game.



  [1]: http://us.battle.net/d3/en/forum/topic/6712951823#16
  [2]: http://www.diablofans.com/topic/71961-the-full-guide-to-obtaining-the-infernal-machine-important-information-on-monster-powers-blizzard-talks-ww-barbs-blizzard-on-energy-twister/