First of all you need to have a account in http://box.net , login to your account.
Now follow 3 easy steps :)
Step1: Go to https://app.box.com/developers/services/edit/
Step2: If step 1 is completed then , Click for configure
Step3: Copy client id and paste in plugin setting
Here is a practical usages of this application in one of our joomla extension “Box.net File picker for joomla”