Go to your Facebook main page , at the menu at the top right choose Account -> Application settings. Find Mafia Wars in the list of applications , choose edit settings. In the application setting window that then pops up go to the Properties tab and you should see "Box: Available(add)" Click "add" to add the box to your private wall. This box will then show up in the boxes tab on your private wall. Go to the boxes tab and click the little pencil and choose to move to wall tab , to move it to your main private wall.'
Note: This is currently not working very well as the box seems to show incorrect information.