The add-in is the complement compatible with the 365 versions. Its placement often varies depending on your Outlook application settings and your build.
The add-in is only visible when composing a new Email.
You can find it here:
In the top ribbon directly:
In the 3 small dots at the end of your ribbon:
From the Insert tab:
If you still cannot find your Add-in, it is possible that your Outlook application encountered an opening issue.Here is the procedure to follow.
Do not hesitate to contact support for more information.