[Tom Bunch]

File -> Print

If you just see copies, pages, and a preview, find the "Show Details" button and click that.

You will see a popup button that will show "Content". Click on that and select Headers & Footer.

You can select the tokens and use Format -> Show Fonts to modify the font size and traits, or use standard key shortcuts like cmd-+, cmd-i. You can also drag in a logo or something, though OmniPlan doesn't scale the images or work very hard at present to prevent some rather hilarious effects.

Note that you can also remove or add tokens to various page locations, and customize the first, odd, or even pages. Or just toss in some static text.