Instructions
Start the game as normal and work your way to the Valley of Spirits after visiting Stark Tower for the third time. The team enters the Forbidden Passage after defeating Ultimo for the second time during the course of this level, and then Dr. Strange appears.
Agree to help Dr. Strange find the Orb of Agamotto and follow the passage until you reach the spinning blades. Pick up the Crimson Dynamo simulator disc on your way through the blades and continue until you reach a door locked from the other side.
Take the other door in the room and follow the path until you reach a treasure chest. Open the chest and take the Orb of Agamotto; Dr. Strange is now unlocked, and he is available to your team at all subsequent SHIELD access points.