In the desktop version of Outlook, the add-in can be accessed through the Quick Access Toolbar. For the add-in to show up you must select an email: 

In Outlook online, you can access the add-in while reading any of your emails, or while drafting a new email. 

If the add-in doesn’t show up there you can access it by clicking on the ellipses “...” in the upper right-hand corner of the email you are working on: 

To keep the Add-in open whenever you are in a message, you can “pin-it” to your view by clicking on the pushpin icon in the Add-In. 

To know more about the Outlook add-in and its possibilities take a look at this article: Outlook add-in