You write an application that has a box. You add the box to the page, then move the box to the page's wall.
For example, I moved the Notes box on my app's page to my wall so you could see it:
http://www.facebook.com/apps/application.php?id=96193636102
Update, with more info:
In general, the way you do this is you add a function to your app that creates a block of fbml and passes it to setFBML(), using your page's ID instead of a user ID as the destination.
I'm planning to do this myself to add some summary statistics to my app's profile page, but that has to wait til after working hours.