How to Use of Installationn Key

This varies between add-ons as they are technically built in different ways, it's always recommended to follow the installation guide for the add-on in question. For the Advanced Outlook Add-in the licensing is described in chapter 2.3 of Advanced Outlook Add-In for M-Files - Installation and Configuration Guide

Thanks Joonas for your assistance. 

How about for an API add-in like Document Compare API 

Please see instructions here: Setting Up M-Files Advanced Document Compare

Please see instructions here: Setting Up M-Files Advanced Document Compare