Ghostbuster for Wii: Stuck!

So Zeddmore and I are in the Mayan exhibit in the museum and we’re in the room where you have to wrangle the big balls (heh) into the purply-glowing goal thingies.

There are three balls, and four goals.

I put the balls in and nothing happens. Do I need a fourth? There’s no way out, nothing to shoot, no more ghosts to bust. Now what?

Yep the fourth ball is invisible in a glass case next to the pyramid. It’s on the right as you face the pyramid as I recall.