BI Attila works as a modfoldered mod on the Steam Release of RTW Gold.
Simply unzip the downloaded file and put the "bi-attila" folder inside your main RTW directory ( where the .exe files are ). Then on Your Steam library:
1.right click on Rome Total War
2. Click "Startup options"
3. paste this text inside the window=> -show_err -nm -mod:bi-attila
4.Start Barbarian Invasion normally.
Enjoy! If You like the Mod please rate it on this site! Thank You!