http://www.moddb.com/mods/medieval-kingdoms-total-war-attila-version

1) Downlod the mod and find the pack file
2) Place that file in the Attila data folder: Library>Application `Support> Steam>Steamapps>Common> Attila> Data
3) go to Library>The Creative Assembly> Attila > scripts and create a txt file with text edit. Name it "user.script.txt"
4) Paste the following into that file: mod (tab) Medieval_Kingdoms.pack;
; has to be written,