There must be a Google account associated with an email address for it to be accepted by Google Apps Script as a reviewer/approver.
Create your free google account here: http://goo.gl/wbP8g1
A non-Google account cannot be editor in Apps Script. The account owner will only get a notification and then a pop-up will ask him to associate his email to a Google account.
If you try to add a non-Google email address as reviewer to your Form Workflow Plus, you will get an error message.