At the bottom right corner there are four squares. The second square has a camera, click on that to activate a pull-down menu. Select the one that looks like footsteps and you will spawn in the bridge. Use WASD or the arrow keys to walk, shift to sprint (doesn't make you go faster very much though), mouse 2 (right button) to look around, V to zoom in (binoculars). Camera mode in the camera menu looks like a regular photo camera. Same thing as walkthrough, I think mouse 1 to take a photo, I haven't used it in a while.