Step 1. From your browser open Outlook Web App just as when you want to check your email over the web.
Step 2. From the Settings menu, choose Manage add-ins
Step 3. On Manage add-in page, click + and choose Add from a URL
Step 4. In the URL field enter https://freebusy.io/outlook/enterprise.xml
If you receive an error that says "The app couldn't be downloaded." then open https://freebusy.io/outlook/enterprise.xml in your browser, download/save the file to your local desktop and repeat this step, but this time choose Add from a file option and supply the file you just downloaded/saved.
If this operation is successful the FreeBusy Outlook add-in will appear on the Manage Add-ins page and in the New Meeting compose window, in the command bar.
If you encounter this error message it means the Exchange/IT administrator for you organization turned off the ability for users to install their own Outlook add-ins.
If this is your situation, please contact your Helpdesk or IT dept and ask that they either turn on your ability to manually install add-ins or request instructions for how the IT admin can install the FreeBusy Outlook add-in on behalf of users.