1) Begin the second mission, Celerium.
2) You will begin the mission looking at a bunker entrance.
3) Kill all of the enemies in front of the bunker.
4) Turn around.
5) Go over to the left cliff wall and walk along it. (You will fall down and become slightly injured).
6) After a while you will come across a cave entrance.
7) Enter the cave.
8) Thor's Hammer will be inside, embedding in a rock on the wall.
9) You will only be able to see the hammer when you use a thermal scope or throw a flash bang.

Here is a video tutorial: