It came with a small Allen wrench. There are 2 Allen screws that hold that knob on. You simply just need to screw it back on. Once you have it back on, you're going to need to screw the part that fell off all the way down till it starts to feel like its going to stop. Then unscrew the 2 Allen's an readjust it to '0' and screw it back down again. If you don't care what it reads, you won't need to do the second step.
In the same envelope that the Allen wrench came in, there was also some directions - I think...