Click on Reset (when it appears) to start the emulation, then click in the display area. My favourite Apple 2 game is Elite, but Drol is fun too! A/Z to go up/down, Space to fire, Left/Right Arrows to go Left/Right (duh!). If you press the Down Arrow you'll activate joystick mode in the game. Since the applet doesn't support a joystick you'll lose control! Press the Up Arrow to regain control. It's got some sort of persistent clicking - I don't know why.
This Java
Apple 2 Emulator written by Steven Hugg. Source code is available from that
page.