Where are the key wardens and where are my keys?
What are the locations of the monsters that can drop the keys and the plans for the infernal machine introduced in patch 1.0.5?
According to Blizzard's article on the Infernal Machine:
Note that these only drop in Torment. The higher the level of Torment, the better the chance of getting one, up to 50% at Torment 6.
The plans are taught to your blacksmith, and only are needed once. After that, you just need the keys from the previous three acts, as using the Infernal Machine consumes it.