Drop the downloaded campaign file into that folder.
Here is the "report" on where to find it and how to stay safe: The Trasamire Campaigns Free Download
Go to the "Custom Campaigns" menu in-game to play. Drop the downloaded campaign file into that folder
The most reputable place to find legacy Age of Empires II mods and campaigns is Age of Kings Heaven (AOKH) . It is a long-standing community archive. The Trasamire Campaigns Free Download