Click on the canvas to move around with WASD/Space/Shift+Space and look by moving the mouse. When you're ready to take a render, press escape and click the Raytrace button.
Use the Precision input to select the distance between each ray on the camera screen. The lower the number, the more rays you get. Be careful, numbers lower than 0.003 tend to produce very long render times.
Clicking on the canvas erases the render.
Hit Shift+D to see camera debug info.