Wait outside until 1:15 or so. I stayed crouched. At 1:15am enter the tower. The person in question should be there.
More concerning is the next stage of the quest appears to be bugged. No way to complete it successfully, even if you pass skill checks.
That's what I did, and it worked for me. The problem I was having was that I would never see someone enter the tower, so i wouldn't go in and if I waited inside no one ever entered. So what I did was I waited out side hiding under the plane wing and used the wait function to advance time to about 1:00am, then I entered the tower at about 1:45 and he was inside.
The second half of the mission can't be completed, the part where you disarm the bomb on the monorail because the train takes off as soon as you go out the door if you try. So don't turn it in until it gets fixed. It wont blow up and you can use the monorail to get to new vegas as long as you don't turn the quest in.