There is a check box on the terrain block options that does it. Unfortunately I can't give you a picture of it because I'm on vacation. You'll have to find it yourself. If the terrain seems to disappear when you turn it on, don't worry. It's just hiding somewhere. I haven't had a chance to figure out where it is centered, but I'm guessing that it's centered at the origin. Move the spawn over to the terrain so the players aren't spawning out in the middle of nowhere.