See instructions below. Also check out the cool new beta version 2.
With the pencil tool, draw a line from top-left towards bottom-right. Press the "Play" button (triangle) and watch line rider go! Experiment with jumps, loops, etc.
Note that lines behave differently depending on the direction you draw them. Lines drawn from left to right act as "floor" and lines drawn right to left act as "ceiling". Line rider won't drop down through a floor but he can miraculously pass up through a floor. Ceilings behave the opposite way.
To make a straight line, place the pen where you want the line to start. Click-hold the left button, then click the right button, then release both. Then move the pen to your desired end point and click the left button.
Use the hand tool to move the drawing area (you can also hold down the space key while using the pencil tool for the same result.
To inspire you, here are some movies of outstanding line rider creations.
Jagged Peak Adventure