Click on the gray rectangle inside the frame.
Keys Q,W,E,... play natural notes starting from C.
Keys 2,3,5,6,7,... play sharps/flats.
Keys A,S,D,... play major chords starting from C.
Keys Z,X,C,... play minor chords starting from Cm.
If notes won't play anymore, hit the spacebar and try again.
This may or may not work for you. It works under Safari on Mac, but it seems not to work on Internet Explorer under Windows. (If you run IE on Windows and the applet does work, please let me know!)