How do I access the desk and conference room computers in the lab?
You had to pick-pocket Warren's pocket when he stands over by the window to get his pen to access his computer, then read his emails to get the password to the conference room computer. He stands over there a lot, but I missed it too, so don't feel bad.
If you're already at the end, and he's already left for the last time though, then I'm afraid that there's nothing that you can do short of just starting a new game on the main menu.