This add-in requires you to have an exchange or outlook.com account - with add-in capabilities enabled. If you're using Office365 and couldn't install (activate) the add-in, please contact your tenant admin.
It allows you to attach files to the email you're composing from Box / Dropbox / Google Drive / OneDrive / OneDrive for Business and SharePoint. You can choose to save the credentials and you will be automatically authorised for these providers next time you're using this add-in. Alternatively you can choose not to save, and authorise every time you want to attach.
Let us know in your comments if you're looking to get more providers!