You password-protect a Numbers ’09 document just like you password-protect a Pages ’09 document:
1. Open the Inspector (View menu/Show Inspector)
2. Click on the Document button in the Inspector (top left), then click “Require password to open”
3. Enter a password, enter it again, and enter a hint. The hint is very important because no one will be able to get you into a password-protected Numbers ’09 document if you forget the password.
That’s all there is to it! From now on, when you go to open that document, you’ll be asked for the password. If you get it right on the first try the document will open right up. I would not recommend saving the password in your keychain because if you do, the system will not ask for the password anymore. That sort of defeats the purpose, doesn’t it.
If you don’t remember the password just keep trying. The hint shows up on your third try so don’t give up after two.
Easy stuff, when you know how.