You are supposed to hack the main control hub at the top of the building. But, this is so impossible and frustrating when the Cybermen are constantly in your way and just end up killing you over and over again!!
You stun them and it only lasts for 10 seconds they constantly turn around giving you no time to hack the damn terminal. WTF!
I figured this out. You basically have to listen to the Doctor at the very beginning. The Doctor works his way up the tower, but as River, you work your way down the tower. You also find another hat! :)
No, that's not the part I'm talking about. It's the building before you go to the cyber factory. And I figured out what to do.
First, you have to wait until the Cybermen reach the top and break both panels of glass surrounding that little room with the computer. Then, lead them all the way back down to the bottom of the building and climb back up. This way you will have enough time to solve the two puzzles (While they are coming back up) in that little room at the top and open the security door.
If you don't do this the Cybermen will just stand up there waiting for you to lead them away. It was very frustrating trying to figure out what I was supposed to do....phew.