Box.net API Key

Visit enabled.box.net and register a developer account. You can then navigate to My projects, and create a new one.

For all the settings, Moodle doesn't care what you put in except for the callback url, which should be $CFG->wwwroot/portfolio/add.php?postcontrol=1.

Box.net will append the relevant authentication tokens to the url (detecting it already contains ? and everything already)