I too have had problems, so I will summarize all of the above.
When I get to level 15 and go down to the bottom of the lift shaft, there is no lift floor so you cannot simply walk onto the ledge. You can jump either, although by detonating a pack and jumping, I was able to get onto the ledge. I then had problems with the game terminating abruptly during the cutscene with Turner and Sophia.
I downloaded ND's save game and this cleared this problem. It allowed to to get as far as the rotating gears - which have to be crossed. Alledgedly, the door at the far end of the gears is supposed to open automatically. It doesn't - Game Over.
I have reported this to Lucas Arts and I received an automated reply (I think) saying to download the 1.2 patch. When executed, the patch says that it's already been installed.
I have e-mailed Lucas Arts back, but I'm not holding my breath.
I still have the receipt for the game and will attempt to return it claiming that it is faulty and throwing the sale of goods act at them using the posts in this thread as evidence.
The Voice of Reason