To help out even though your really supposed to contact themehouse from the UI.X thread,
The big download button at the top in the download area is for the add-on and for installing the theme via the add on. Your supposed to go to appearance > themehouse styles and click that then install it after you put in your api key in themehouse Xenforo Settings.
If you want to install it manually you can do that too by clicking the smaller download button below the big one. The add on is still required however but you'll import the xml file the old fashioned way which is the difference (it is provided in the smaller download button).