Default controls:
F1 - reset
F2 - select
F3 - black-and-white mode
F4 - color mode
F5 - left player - amateur mode (difficulty B)
F6 - left player - professional mode (difficulty A)
F7 - right player - amateur mode (difficulty B)
F8 - right player - professional mode (difficulty A)
Arrow keys - left joystick directions
Space bar - left trigger
Mouse wheel - left paddle A
Right mouse button - switches to paddle mode. The cursor becomes invisible, and mouse movement simulates the left paddle (A).
Left mouse button - trigger for left paddle A
More control options/implementations can be viewed in JStella's configuration dialog.