Navigate to Properties > Local Files > Browse Local Files.
If it can't find the game, from your Steam library, right click on Age of Empires II: HD Edition and click on Properties. Then, open any one of the AoE 2 game lobbies, such as the New Player Lobby (NPL).
This will not overwrite or destroy any files that AoE2 HD uses it simply adds folders and files. Download the userpatch from and run it.
This Reddit thread gives detailed instructions on how to do it, but I will outline how to do it here, assuming you already have AoE2 HD installed on Steam: AoE2 HD can be played on Voobly, but you need to install the compatibility patch.
