Minor Penny, can you make the box.net config screen (where you put the key) also print this text, substituting the correct URL so that people can just copy the URL, click on the link, log in to Box.net and paste:
To configure Box.net, visit http://enabled.box.net/my-projects and log in. Under My Projects you will need to create one new project for each Moodle site. The only setting that matters is the callback url, which should be $CFG->wwwroot/portfolio/add.php?postcontrol=1. You can put anything you like for the other settings. Save it and you're done!