In Pages ’09, look at the Document section of the Inspector. You get to the Document section by clicking the button at the top left of the Inspector. See below.
(If the Inspector isn’t showing, look in the View menu for “Show Inspector” or just click the blue Inspector button in the Toolbar.)
At the very bottom of the Document section of the Inspector you’ll see “Require password to open.” Check that box, choose a password, enter it a second time, and you’re done. It would be good to include a hint because there’s no way to pry the document open without the password.
Here’s a picture of what you’re looking for in the Inspector.
Your protected document will get a new icon:
When you try to open it you’ll be asked for the password. Enter it successfully and the document opens right up. Get it wrong, and the document stays closed.
Nothing to it when you know how.