Hey, sorry for the late response. Are you trying to make a mod in the Steam version or the old Forgotten Expansion mod that is available for the CD version?
If it's for the Forgotten Expansion mod for the CD version, I would double check the file paths you are using when you start AGE. I believe the filepaths to the .dat file, the DRS folder, and the language files should be in this folder: C:\Program Files (x86)\Microsoft Games\Age of Empires II\Games\Forgotten Empires\Data. You might have to modify them if your game is installed in a slightly different location.
If it's for Steam, I believe you copy your .slp files into the C:\Program Files (x86)\Steam\SteamApps\common\Age2HD\resources\common\drs\graphics folder. You can also copy them into the gamedata_x2 folder for good measure. I haven't done any modding for the Steam version, so I'm not as certain about which folder to copy them to. Steam doesn't use a gamedata_x1_p1.drs file.
I hope this helps. If not, let me know and we'll see if we can figure out what isn't working.
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