After you open the locket for Jess --
-- she runs off and leaves a note that sends you into the tunnel where Jess was hiding. And as Nancy says, it's dark, so you need to turn on the generator to get some light.
The generator is that big machine in the corner next to the entrance to Jess's tunnel. Click on it to reveal the puzzle -- and as always, read to the end of this hint for a picture showing you how to solve it.
Your goal is to get the little ball at the upper left to the square with the hole in it, about four squares to the right. You move the ball with the mouse; move it till you see an arrow pointing the way you want the ball to go and click.
At first this looks like an easy puzzle; move the ball to the right, and the little doors fold down as you pass. But ... hmm.
In order to reach the square with the hole, you have to pass over it, which closes the little door so you can't reach it. Moving left opens the door again ... but now the door is a wall and you can't reach the square with the hole.
But now that the door's open, maybe there's a way to reach it from the other side.
Actually, it's a little more complicated than that. What you need to do is go all the way around the maze and approach the hole from the left, which opens the door; then go all the way around again to reach it from the right and drop down into it.
You can plan this out completely before you start -- notice the little scratch marks that show which way each door opens -- or you can just set out and see what works for you.
Stymied? Here's a screenshot that may help. Follow the red line going in to open the door, then follow the blue line to reach it from the other side.