It's a torrent file, and it's useless unless you actually have a torrent program download it with.
If you don't have one, I'd suggest you to download utorrent.
Once you have a torrent program installed, and you press the link, it should say Open with, or save instead.
You just open it with the torrent program, let it download so it reaches 100%, then you run the .exe file that you just downloaded and it should install the map pack.