If you installed WoW on the default path, then you want to place all of the files in
C:\Program Files (x86)\World of Warcraft\_retail_\Interface\AddOns\Gathermark
The easiest way to go about this would be to simply download the files as a zip folder, and extract them to the given path

The main part of the addon is a frame on the left of the screen that shows you when you started mining and how many of each item you've looted since. Additionally, there is a slash command called /gmark or /gathermark (either works) that will show you the same info in the chat window, with a small amount of additional data.