The best way to start is to click one of the preset buttons (under the word "PRESETS"). Then start varying the parameters to get a feel for what they do. mutate and random provide alternative methods for exploring the sound space. Check out sfxr and Dr Petter's site for a description of the parameters and some theory, etc. Have fun, and be sure to check out the Beads project - BP 040409.
Source code: Jsfxr
Problems? If it doesn't seem to work - try reloading the page. If that doesn't work, you can download the Jsfxr processing application and run it directly from Processing. A recent copy of beads.jar is included, which you'll have to drop into libraries\Beads\library in your Processing sketches folder. (You'll also need controlP5.)