Looking at your post I think PLK's correct. The directory you want to place the new files in is:

C:\Program Files\Activision\Rome - Total War\Data\UI\BARBARIAN\BUILDINGS

I've been struggling with a similar problem to this for the last few hours, but mine had a different solution.

My problem was with the file name of the building card, I was naming it after the building name that appears onscreen rather than the one listed in the levels line in the export_descr_buildings entry.

You need to copy the UI folder out into C:\Program Files\Activision\Rome - Total War\Data\ and rename the corresponding .pak file so that the game cannot find it, and looks in the UI folder.
I've not found it necessary to rename the paks, as I understand it, if the game can't find a file within a pak it will look in the corresponding directory structure within the data folder for it. This is why conversion mods don't require the paks depacked before you can use them.