He stuck it in his pant leg and up into a backpack. He acted like he had a limp and limped all the way through a neighborhood, into the campus, into the building and up on the roof. There was a stairway that led directly onto the roof. The whole thing is caught on cameras. He climbs over this small wall next to the public roof area and hides for about 20 minutes where he assembles something with the gun, and then goes to his perch and makes the shot. Then he runs across the giant roof area to the corner farthest away from where Kirk was, throws the gun down to the grass, jumps down, and then walked over to a wooded area across the street where he dumped the gun.
Since it was caught on cameras they immediately searched the wooded areas because they saw him walk in there. This video does a good walkthrough.
The guy is a bit of a dildo but the walkthrough makes sense if you watch all the camera videos. For me, its very strange how easy it is to get up on the roof and get down from it. Its tailor made for a snipers position. Not a difficult maneuver and certainly wouldnt require a bunch of extensive training or anything. The main question for me is how he got the idea to use the roof. Could be as simple as he had been up there before and then heard where Kirk would be setting up and realized he could likely pull it off without having to do anything too complicated.