First off, give the mod a few minutes to download, you can gauge this by the 'Download Bar' at the bottom of Steam. Make sure you have 5GB free for whereever you are downloading it also.

Once you have downloaded it successfully, go back to your library, and follow these steps.
Right-click on Empire At War,
Go down to Properties,
Find Launch Options, and paste "Modpath=..\..\..\workshop\content\32470\3534435923" into the launch options,
Launch the game, and see if it worked!

Alternatively, here is an explanation in video form.
https://www.youtube.com/watch?v=Uzm_nQbVkzg&ab_channel=StarGhostss