Paste the map files you copied earlier into that folder.
Well, I've got the same case and I haven't found an answer to that yet!! *NOTE* if you have an expansion pack such as the War Chiefs you will have to place the map files into the folder named 'RM2'. Once open there will be a file named 'RM'. Place maps to C:\Users\%your windows login%\Documents\My Games\Age of Empires 3\Scenario\ sound and etc. So I was doing some browsing around in my age of empires files, trying to figure out what to do, when I realized theres a history channel video in my age of empires folder wtf.
place in steamapps\common\Age of Empires III\bin\sounds etc. Longer ages show the true advantages and disadvantages of cultures and demand fThis is a custom scenario I made with different objectives to complete.