Single player: Up Arrow to go up, Down Arrow to go down. Multiplayer: For Player 1, W to go up and S to go down, while for Player 2 Up arrow to go up and Down arrow to go down.
The PC-Controlled platform is sometimes slow but that's on purpose, or otherwise you'd never score a point ;) You can toggle Trace mode on and off by pressing T, it allows you to see the algorithm in action.
