Assuming you installed BG2 into the default folder, the override folder is located in this directory,

<font color="white">C:\Program Files\Black Isle\BGII - SoA\override</font>

Simply copy the unzipped files and paste it into this folder.

you just need to put the files in the override folder - my download came with a readme that helped but it should just be the case of putting the unzipped stuff in the override folder then going to those places and meeting the merchants.