After you finish sequence 12, you'll be brought back to present day and have to watch a few cut-scenes. I won't spoil any of the story, but afterwards, you are told to return to your animus, and are told that you have level 3 security access from then on. Instead of going back to your animus to finish the story, walk around the main room of Abstergo and hack all your colleagues computers. This should bring you to about 7-8 computers hacked. Afterwards, head around the edge of the main circular room, and head through the various security doors you see. There's usually a few PC's past each of them. Since there's 33 possible computers to hack, and you only need 15, it shouldn't be too hard to locate them.