Requirements
Age of Empires II: Forgotten Empires is an unofficial expansion for Age of Empires II: The Conquerors. You this need the Age of Empires II “Age of Kings” and the expansion pack “The Conquerors” before you can install Forgotten Empires.
Install
Make sure to have Age of Empires II: The Conquerors installed
Download the installer exe | zip
Unzip the file into your Age of Empires II Installation folder (typically C:\Program Files\Microsoft Games\Age of Empires II\) (example picture)
Select your language
Click Install