To add the mod you need to follow installation procedure from the first post: https://forums.civfanatics.com/threads/welcome-to-the-mp-sub-forum.248164/
I don't know if this mod is compatible with the Steam version (and its multiplayer).
If not, you need to change the game to its original version by right click -> Properties -> Betas -> original_release_unsupported .
You also need this version if you want to use Direct IP.
Thanks for you quick reply. I did try to follow the instructions in the thread, but I don't understand how to "Extract the files contained in the zip in ...your Civ4 folder...\BTS\Mods"
I play the game using Steam, so I don't think I can physically download the game to add to the folder. Also I am unsure what files to add to the folder: do I add everything from the download?
Assuming I cannot use Steam, how do I change the game to its original version by right click? If I right click on the Steam version in the library, nothing happens. If I buy a PC download for Steam, will this work?
I love Rhyse and Fall mod and want to play it MP, but if this is not possible, does Civ VI have anything similar that you know of?
Seems such a shame that this great game was not developed!
best wishes,
Andy