1. Download the mod.
2. Use the Unarchiver to unpack the files. Place the .packs in your data folder.
http://the-unarchiver.en.softonic.com/mac
3. If the file is a .exe use Paul the Tall's shogun wrapper:
http://paulthetall.com/shogun-2-total-war-mac/.
Just right click on the .exe file and click open with => select wrapper.
4. When you are installing a .exe, don't install it to your Empire App, instead install it to a separate folder. Then just take the .packs and place them in your data folder,
5. If the mod contains any startpos.esf or scripting.lua files place them in your campaign -> main folder. (remember, don't replace the startpos.esf's that are in there, just rename the old ones, things like "startpos.esf vanilla". Whatever startpos you want to actually use, just name that one simply "startpos.esf"
Running a Mod:
1. Double click on Empire.
6. Return to your Empire launcher, and click on the "Advanced" tab.
7. In advanced options list your .packs like this: mod "AUMgraphics.pack"; mod "unit graphics.pack";
8. Play!
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NTW NOTE: If you are trying to run mods on NTW, don't use the quotes. Other than that the process is pretty much the same…as far as I can tell.
Additional Notes:
Sometimes you may experience crashes if you have mod .packs in your data folder that you are not using. At times it appears some continue to run and cause conflicts. What I do is keep a separate folder containing sub folders for each of my mods. When I want to use a particular mod I just put it's .packs into my data folder; and remove the mod .packs I am not using.
To remove a .pack from the data folder, you must drag it into the trash, then drag it from there into your mod folder. If you do not do this and try to drag it directly from your data folder to your mod folder you will create an alias and the .pack will remain in the data folder.