I'm guessing you don't have any other mods installed and are using the native module. If either of these assumptions are wrong, these directions may not work.

1) Download the file
2) Extract the file somewhere you can find
3) Go to your Warband folder and double click to go into the Modules and then go into the Native directory
4) copy the item_kinds1.txt and module.ini file to somewhere safe - this is your backup in case something goes wrong
5) open the item_kinds1.txt file that was in the archive you downloaded from here in notepad and select everything and copy it to the clipboard and close this file
6) open the item_kinds1.txt file that is in the Native directory with notepad and paste what you just copied at the bottom of the file without overwriting anything.
7) go to the top of this file and find a number, should be the second line. For example, 600 all by itself
8) Add 47 to whatever that number is, for example change 600 to 647 and save this file
9) Open the module.ini file that's in the Native directory and add the following somewhere near the bottom and save
Code:
load_module_resource = Addonay
10) Copy every file with the extension .dds for the files you extracted into the Native\Textures directory
11) Copy the file Addonay.brf from the file you downloaded into the Native\Resource folder

That's it, should be ready to play. Give the markets a week or so for their stock to refresh