Ok I know that a lot of people are having a ton of trouble using mods with the Steam versions of totalwar it's not just RTW but all of the TW games. However after much combing of the forums I have managed to get enough information from various places to be able install most mods. I would like to remind people that I am running this on Vista but from what I have read you should be able to do the same with Windows7.
First make a copy of your vanilla files.
Download the current version of FATW here http://www.twcenter.net/forums/showthread.php?t=365719
Now you have the file FATW_TNS_2.6_Full to install right click on the file and click Run As Administrator.
The installer will give you a directory to save into but this is for shop bought games only. You will have to change it to C:\Program Files\Steam\steamapps\common\rome total war gold Important do not put into the BI folder just click onto the RTW folder the installer will do the rest.
Once the Installer has finished DO NOT RUN the game just yet first go to C:\Program Files\Steam\steamapps\common and right click on the rome total war gold folder and click on properties then go to the Security Tab under a box containing all your user names you will see the button Edit click on this.
A new window will open similar to the previous one but you will notice that at the bottom of the window the list of options called Permissions for Home (or whatever user name your using) and you will now be able to tick the Allow/Deny permissions. Now Click the full control box at the top and all the boxes below should automatically tick also. Now click OK and a small window will appear running through all the files that it is changing the permissions to. Once this finishes just click ok to exit all the boxes.
Now go the Desktop and Delete the FATW Shortcut as this does not work for Steam Users.
Bottom right corner of your Window you should see your programs currently running, Double click on the Steam Icon and once Steam opens go to Games at the top of the window and click on it and select View Games Library, Now right click on the Rome: Total War Gold Edition and go to properties. In this window select Set Launch Options. This box should be empty of text. Now Input -show_err -nm -mod:bi/ci in this window and select Ok. Now press close and then little X in the top right corner this should take you back to your desktop.
Right Click the Steam Icon and select Rome: Total War Gold Edition to launch the game when the box appears asking if you want to play Rome or BI just click on The BI version and click OK.
I have done this myself and FATW works fine on My comp. Hope this Helps guys/Gals.