There are a couple of options here. First, you can use the mouse position to determine paddle position; either measured across the range of the video window or across the range of the screen.
Second, you can use the keypad as a coarse joystick; this is easier to deal with in a lot of situations.
Buttons default to command for button zero and option for button 1 (shift for button 2, for what it's worth, which may be a lot depending on what you're up to…). There's an option to map button zero to the mouse button, though.
