There's no unlocks (other than the hand-made levels). You can turn hard mode on the generator on by clicking the slider (not the word "Hard") at the top of the generator screen.

Thanks Boksha. I had to click the slider like 20 times before it would move, but it finally did. Weird.

I just figured this out myself. At least on my machine, in the game, I always have to click at the top edge of the cells in order to select them. The same is true for the Easy/Hard slider -- if I click *above* the slider, it works.