I think I'm missing a nut.

Perfect! And that's not a nut, but it's a bolt. The nut is the installer. Ha!
I was waiting for someone to call it a bolt. LOL I'm not familiar with a 225 or any other 6 banger but what keeps the balancer from flying off without the bolt? I would think you would need the bolt to install it but why then would you remove the bolt afterward?